Valve  Gears  and  Indicators 


A  Manual  of 

PRACTICAL    INSTRUCTION  IN  VALVE-SETTING,   USE    OF    INDICATORS,     AND 
OTHER      DETAILS      OF     STEAM     ENGINE     OPERATION     ES- 
SENTIAL TO    EFFICIENCY    AND    ECONOMY 


PART    I— VALVE    GEARS 

By  WALTER  S.  LELAND,  S.B. 

Assistant  Professor  of  Naval  Architecture.  Massachusetts 
Institute  of  Technology 


PART    II— STEAM    ENGINE    INDICATORS 

By  CARL  S.   Dow,   S.B. 

American  Society  of  Mechanical  Engineers.     With  B.  F.  Sturtevant  Company, 
Boston,  Mass, 


ILLUSTRATED 


CHICAGO 

AMERICAN    SCHOOL  OF  CORRESPONDENCE 
11  1908 


Wittlttl. 


COPYRIGHT  1907  BY 
AMERICAN  SCHOOI,  OF  CORRESPONDENCE 


Entered  at  Stationers'  Hall,  London 
All  Rights  Reserved 


Foreword 


recent  years,  such  marvelous  advances  have  been 
made  in  the  engineering  and  scientific  fields,  and 
so  rapid  has  been  the  evolution  of  mechanical  and 
constructive  processes  and.  Irii&thodsj*  that  a  distinct 
need  has  been  created  for  a  series  of  practical 
working  guides,  of  convenient  size  and  low  cost,  embodying  the 
accumulated  results  of  experience .  and  the  most  approved  modern 
practice  along  a  great  variety  of  lines.  To  fill  this  acknowledged 
need,  is  the  special  purpose  of  the  series  of  handbooks  to  which 
this  volume  belongs. 

d,  I11  the  preparation  of  this  series,  it  has  been  the  aim  of  the  pub- 
lishers to  lay  special  stress  on  the  practical  side  of  each  subject, 
as  distinguished  from  mere  theoretical  or  academic  discussion. 
Each  volume  is  written  by  a  well-known  expert  of  acknowledged 
authority  in  his  special  line,  and  is  based  on  a  most  careful  study 
of  practical  needs  and  up-to-date  methods  as  developed  under  the 
conditions  of  actual  practice  in  the  field,  the  shop,  the  mill,  the 
power  house,  the  drafting  room,  the  engine  room,  etc. 

C.  These  volumes  are  especially  adapted  for  purposes  of  self- 
instruction  and  home  study.  The  utmost  care  has  been  used  to 
bring  the  treatment  of  each *  subject  within  the  range  of  the  com- 


179744 


mori  understanding,  so  that  the  work  will  appeal  not  only  to  the 
technically  trained  expert,  but  also  to  the  beginner  and  the  self- 
taught  practical  man  who  wishes  to  keep  abreast  of  modern 
progress.  The  language  is  simple  and  clear;  heavy  technical  terms 
and  the  formulae  of  the  higher  mathematics  have  been  avoided, 
yet  without  sacrificing  any  of  -the  requirements  of  practical 
instruction;  the  arrangement  of  matter  is  such  as  to  carry  the 
reader  along  by  easy  steps  to  complete  mastery  of  each  subject; 
frequent  examples  for  practice  are  given,  to  enable  the  reader  to 
test  his  knowledge  and  make  it  a  permanent  possession;  and  the 
illustrations  are  selected  with  the  greatest  care  to  supplement  and 
make  clear  the  references  in  the  text. 

C.  The  method  adopted  in  the  preparation  of  these  volumes  is  that 
which  the  American  School  of  Correspondence  has  developed  and 
employed  so  successfully  for  many  years.  It  is  not  an  experiment, 
but  has  stood  the  severest  of  all  tests- — that  of  practical  use — which 
has  demonstrated  it  to  be  the  best  method  yet  devised  for  the 
education  of  the  busy  working  man. 

C.  For  purposes  of  ready  reference  and  timely  information  when 
needed,  it  is  believed  that  this  series  of  handbooks  will  be  found  to 
meet  every  requirement. 


Table    of    Contents 


PART  I -VALVE  GEARS 
SLIDE  VALVES '.  ... ..     '-  .      ..'       .    Page,  3 

Plain  Slide  or  D  Valve — Eccentric — Inside  and  Outside  Lap — Angular 
Advance  —  Admission — Cut-Off— Release — Compression — Lead — Angle 
of  Lead — Inequality  of  Steam  Distribution — Displacement  of  Valve 
and  Piston — Compensation  of  Cut-Off — Rocker. 

VALVE-DESIGNING  AND  VALVE-SETTING  ..      .       »    ....*.       .    Page  16 

Valve  Diagrams  (Zeuner's) — Area  of  Steam  Pipe — Width  of  Steam 
Port — Width  of  Exhaust  Port — Width  of  Bridge — Point  of  Cut-Off — 
Lead — Putting  Engine  on  Center — Setting  Valve  with  Equal  Lead — 
Setting  Valve  for  Equal  Cut-Off — Modifications  of  the  Slide  Valve 
(Piston  Valve,  Double-Ported  Valve,  Trick  Valve) — Balanced  Valves — 
Link  Motion  (Stephenson,  Gooch). 


RADIAL,  DOUBLE,  AND  DROP  CUT-OFF  VALVE  GEARS  .       .      . .    Page  49 

Hackworth  Gear — Marshall  Gear — Joy  Gear — Walschaert  Gear — Ad- 
justable Eccentrics — Meyer  Double  Valve — Reynolds-Corliss  Drop  Cut- 
Off  Gear — Safety  Cams — Brown  Releasing  Gear — Greene  Gear — Sulzer 
Gear — Corliss  Valve  Setting. 


PART   II-STEAM  ENGINE   INDICATORS 
TYPES  OF  INDICATORS      -.;;;".       .       .       .  ;    .       .       .       .    Page   3 

Definition  of  Terms  and  Explanation  of  Principles — Watt's  Diagram 
of  Work — Thompson  Indicator — Crosby  Indicator — Tabor  Indicator — 
Reducing  Motion — Pantograph — Brumbo  Pulley. 

USE  OF  INDICATOR  DIAGRAMS  .   ....  .  -  -V  .»   .   .   .  Page  22 

Determination  of  Indicated  Horse-Power — Mechanical  Efficiency — 
Steam  Distribution — Mean  Effective  Pressure — Piston  Speed — Table  of 
Engine  Constants — Brake  Horse-Power — Prony  Brake — Rope  Brake — 
Finding  Area  of  Cards — Planimeter — Thermal  Efficiency — Theoretical 
Indicator  Diagram — Atmospheric  Line — Admission  Line — Steam  Line — 
Point  of  Cut-Off — Expansion  Curve — Point  of  Release — Exhaust  Line — 
Back-Pressure  Line — Point  of  Exhaust  Closure — Compression  Curve — 
Zero  Line — Clearance  Line — Drawing  the  Theoretical  Card — Cards  for 
Compound  Engines — Combined  Diagrams — Horse-Power  of  Compound 
Engines — Defects  Revealed  by  Card  (Events  Too  Early  or  Too  Late, 
Unequal  Work  at  Cylinder  Ends,  etc.) — Steam  Consumption. 


INDEX   .       ,.,       .        ,      -.       •"•*->'•  :  ••-       •        •        •    Page  59 


VALVE  GEARS 


Steam  enters  the  cylinder  of  the  engine  through  ports  which 
must,  in  some  manner,  be  opened  and  closed  alternately,  in  order 
to  admit  and  exhaust  the  steam  at  the  proper  time.  To  accom- 
plish this  purpose  a  valve  is  moved  back  and  forth  across  the  port 
openings.  A  complete  understanding  of  the  valve  and  valve  gear 
is  essential  to  the  engineer  as  well  as  to  the  designer,  for  even 
though  a  valve  be  properly  designed,  its  economy  may  be  seri- 
ously impaired  by  improper  setting.  The  design  and  adjust- 
ment of  these  valves  plays  a  very  important  part  in  the  efficient 
action  of  the  steam  engine. 

o 

The  term  " valve  gear"  includes  the  valve  or  valves  that 
admit  steam  to  and  exhaust  it  from  the  cylinder  of  the  engine, 
together  with  the  mechanism  from  which  the  valves  derive  motion. 

o 

There  may  be  a  single  valve  to  regulate  admission  and  exhaust,  or 
there  may  be  a  double  set  of  valves;  one -set  to  admit  the  steam  at 
each  end  and  another  to  release  it.     The  valve  may  have  a  plain 
reciprocating  motion,  moved 
by  a  rod,  or  it  may  be  opened 
by  some  device  that  lets  go  at 
the  proper  time,  allowing  the 
valve  to  drop  shut  under  the 
influence  of  counter  weights, 
springs  or  vacuum  dashpots. 
To  the  first  class  belong  the 
plain  slide  valve  and  its  modi- 
fication of  piston  valve,  gridiron  valve,  etc.;  to  the  second  belong 
such  valves  as  the  Corliss,  Brown,  and  others. 

The  simplest  type  of  valve  is  the  plain  slide  or  D  valve  as 
shown  in  Fig.  1. 

In  this  figure  Y  is  the  valve,  R  the  valve  rod,  K  the  exhaust 
cavity,  P  and  P*  the  steam  ports,  E  the  exhaust  port,  AB  the  valve 

seat,  and  DM  the  bridges  of  the  valve  seat.     The  valve  seat  must 

o 

be  planed  perfectly  smooth,  so  that  pressure  on   the  valve  will 


Fig.  1. 


VALVE  GEARS 


make  a  steam  tight  fit,  and  cause  as  little  friction  as  possible  when 
the  valve  slides.  Furthermore,  the  length  of  the  seat  AB  must  be 
a  little  less  than  the  distance  from  the  extreme  right-hand  posi- 
tion of  the  right-hand  edge  of  the  valve  to  the  extreme  left-hand 
position  of  the  left-hand  edge  of  the  valve.  This  allows  the  valve  at 
each  stroke  slightly  to  over  travel  the  seat,  thus,  keeping  it  always 
worn  perfectly  flat  and  smooth.  If  the  valve  seat  were  not  raised 
slightly  above  the  rest  of  the  casting,  or  if  it  were  too  short,  the 
constant  motion  of  the  valve  would  soon  wear  a  hollow  path  in  the 
valve  seat,  and  it  would  cease  to  be  steam  tight. 

Eccentric.     The  valve  usually  receives  its  motion  from    an 
eccentric  which   is   simply  a  disc,   keyed  to  the  shaft  in  such  a 


ivfc  i 


Fig.  2. 

manner  that  the  center  of  the  disc  and  the  center  of  the  shaft  do 
not  coincide.  li;  is  evident  that  as  the  shaft  revolves,  the  center 
of  this  eccentric  disc  moves  in  a  circle  about  the  shaft  as  a  center, 
just  as  if  it  were  at  the  end  of  a  crank.  The  action  of  the  eccentric 
is  equivalent  to  the  action  of  a  crank  the  length  of  which  is  equal 
to  the  eccentricity  of  the  eccentric  (the  distance  between  the  center 
of  the  eccentric  and  that  of  the  shaft). 

Fig,  2  represents  the  essentials  of  an  ordinary  eccentric.  O 
is  the  center  of  the  shaft,  O  the  center  of  the  eccentric  disc  E9  and 
S  is  a  collar  encircling  the  eccentric  and  attached  to  the  valve 
rod  E. 


VALVE  GEARS 


As  the  eccentric  turns  in  the  strap,  the  point  O  moves  in  the 
uotted  circle  around  O',  and  the  point  A  also  moves  in  a  circle. 
When  half  a  revolution  is  accomplished  the  point  O  will  be  at 
O",  the  point  A  will  be  at  A",  and  the  eccentric  strap  and  valve 
rod  will  be  in  the  position  indicated  by  the  dotted  lines. 

Since  the  revolving  shaft  transmits  motion  to  the  valve 
through  the  eccentric,  it- will  be  necessary  to  study  the  relative 
motions  of  the  crank  and  eccentric  in  order  to  get  a  clear  idea  of 
the  steam  distribution. 

The  distance  of  the  center  of  the  eccentric  from  the  center 
of  the  shaft  (GO'  in  Fig.  2)  is  known  as  the  eccentricity,  or  throw, 
of  the  eccentric.  The  travel  of  the  valve  is  twice  the  eccentricity. 

Valve  without  Lap.  Fig.  3  shows  a  section  through  the 
steam  and  exhaust  ports  of  an  engine,  together  with  a  plain  slide 


Fig.  3. 

valve  placed  in  mid-position,  and  so  constructed  that  in  this  posi- 
tion it  just  covers  the  steam  ports  and  no  more.  A  valve  is  in 
mid-position  when  the  center  line  of  the  valve  coincides  with  the 
center  line  of  the  exhaust  port. 

Fig.  1  shows  the  same  valve  drawn  to  a  larger  scale. 

Suppose  the  valve  is  moved  a  slight  distance  to  the  right; 
the  port  P  (see  Fig.  1)  is  then  uncovered  and  opened  to  the  live 
steam  which  enters  the  cylinder  and  causes  the  piston  to  move. 
Since  the  two  faces  of  the  valve  are  just  sufficient  to  cover  the 
steam  ports,  it  is  evident  that  as  the  port  P  opens  to  live  steam, 
the  port  P'  opens  to  the  exhaust.  The  ports  are  closed  only  when 
the  valve  is  in  mid-position.  This  allows  admission  and  exhaust 


VALVE  GEARS 


to  continue  during  the  whole  stroke.  With  such  a  valve  there  is 
no  expansion  or  compression;  the  indicator  card  would  be  a  rec- 
tangle, and  the  M.  E.  P.  would  be  equal  to  the  initial  steam  pressure, 
assuming  no  frictional  losses  in  the  steam  pipe  or  condensation  in 
the  cylinder. 

For  a  theoretical  discussion  of  valve  motion,  it  is  assumed  that 
the  eccentric  rod  moves  back  and  forth  in  a  line  parallel  to  the 
center  line  of  the  engine.  This  is  not  the  case  in  practice,  for  the 
eccentric  rod  always  makes  a  small  angle  with  the  center  line,  just 
as  the  connecting  rod  does,  but  the  eccentricity  is  so  small  in  com- 
parison with  the  length  of  the  eccentric  rod  that  the  angularity  of 
the  eccentric  rod  is  very  much  smaller  than  the  angularity  of  the 
connecting  rod,  and  its  influence  may  be  neglected  without  appre- 
ciable error. 

I 


J 


Fig.  4. 


When  the  valve  shown  in  Fig.  3  is  in  mid -position,  the  crank 
is  on  dead  center,  the  eccentric  is  set  at  right  angles  to  it,  and  the 
piston  is  just  ready  to  begin  the  stroke. 

Fig.  4  shows  the  relative  positions  of  crank,  piston,  eccentric 
and  valve  when  the  crank  has  made  a  quarter  turn  or  the  piston 
has  moved  to  half  stroke.  The  eccentric  is  now  in  its  extreme 
position  to  the  right,  the  valve  has  its  maximum  displacement  and 
both  the  steam  and  exhaust  ports  are  wide  open.  The  valve  will 
not  close  again  until  the  piston  has  reached  the  end  of  its  stroke. 

This  type  of  valve  is  used  only  on  small  and  unimportant 
engines,  and  since  it  allows  no  expansion  of  the  steam,  is  very 
uneconomical.  Furthermore,  it  will  be  seen  that  this  valve  opens 
just  after  the  stroke  begins,  which  is  impractical,  for  it  means  that 
the  piston  has  begun  its  stroke  before  the  full  steam  pressure 


tl 
S  i 


VALVE  GEARS 


reaches  it;,  which  will  cause  an  inclined  admission  line  on  the  indi- 
cator diagram. 

Valve  with  Lap.  If  the  face  of  the  valve  is  made  longer 
than  showjjjn  Fig.  iJso  that  in  mid-position  it  overlaps  the  steam 
ports,  ive  shall  have  a  valve  such  as  shown  in  Fig.  5.  The 
amount  that  the  valve  overlaps  the  steam  ports  is  called  the  lap  of 
the  valve.  In  Fig.  5,  DI  is  the  inside  lap,  and  OC  the  outside  lap. 
It  will  at  once  be  seen  that  both  the  admission  and  exhaust  ports  may 
remain  closed  during  a  part  of  the  stroke,  thus  making  expansion 


Fig.  5. 

and  compression  possible.  It  is  also  evident  that  steam  cannot  < 
be  admitted  until  the  valve  uncovers  the  port  by  moving  a  dis- 
tance from  mid-position  equal  toT)C.  Admission  continues  until 
the  valve  returns  to  such  a  position  that  the  outer  edge  of  the 
valve  again  closes  tho  "port.  Release  will  begin  when  the  inner 
edge,of_the  inside  lap  begins  to  uncover  the  port. 


(Fig.  0  represents  a  valve  with  lap,  at  the  point  of  admission.  | 
Since  the  valve  must   move  a  distance  equal  to  the  outside  lap 
before  admission  can  take  place,  it  is  evident  that  the  eccentric  can 

no  longer  be  at  right  angles  to  the  crank  at  the  beginning  of  the 

stroke;  but  must  be  ahead  of  the  right-angle  point  by  an  amount  \ 
equal  to  AGO.     The  angle  AGO  is  known  as  the  angular  advance. 
The  maximum  displacement  of  the  valve  is  attained  when 
the  eccentric  is  horizontal  as  shown  in  Fig.  7.     In  this  position 


8 


VALVE  GEARS 


both  the  steam  and  exhaust  ports  are  wide  open,  and  any  further 
motion  of  the  piston  will  cause  the  valve  to  move  toward  its  mid- 
position. 

Admission  continues  until  the  valve  returns  to  the  position 


Fig.  6. 

shown  in  Fig*  8.  Here  the  outside  lap  just  closes  the  left-hand 
steam  port,  cut-off  takes  place,  and  the  steam  already  in  the  cylin- 
der begins  to  expand.  As  the  valve  continues  to  move  toward  the 
left,  the  left-hand  inside  lap  begins  to  uncover  the  left-hand  port 
and  releases  the  steam  at  the  position  shown  in  Fig.  100 

The  dotted  lines  of  Fig.  7   show  the  valve  in   its  extreme 


Fig.  7. 

position  to  the  left.  Any  further  movement  of  the  piston  will 
cause  it  to  return  toward  mid-position. 

The  dotted  position  of  crank  and  eccentric  in  Fig.  10  shows 
the  valve  returned  to  the  point  of  compression,  which  continues 
until  the  conditions  of  Fig.  G  are  again  reached  and  the  opening- 
valve  allows  steam  again  to  enter  the  cylinder. 

This  process  has  been  traced  step  by  step  for  one  end  only; 
let  us  now  consider  what  is  happening  at  the  other  end. 


VALVE  GEARS 


9 


Admission  is  the  point  at  which  the  valve  opens  to  admit 
steam  to  the  cylinder.  Cut=off  is  the  point  at  which  the  valve 
closes  to  cut  off  the  admission  of  steam.  Release  is  the  point  at 
which  the  exhaust  is  opened ;  and  Compression  is  the  point  at 
which  the  exhaust  is  closed. 


Fig.  8. 


While  the  crank  is  moving  from  the  position  shown  in  Fig. 
6  to  that  of  Fig.  8,  steam  is  being  admitted  to  the  head  end  and 
being  exhausted  from  the  crank  end.  The  inside  lap  being  less 
than  the  outside  lap,  causes  the  exhaust  to  continue  longer  than 
the  admission. 

Fig.  9  shows  the  relative  positions  of  crank,  eccentric  and 
valve  when  the  exhaust  closes  on  the  crank  end  and  compression 


Fig.  9. 


V-t 


begins.     Between  these  two  positions  the  steam  is  expanding  in 
the  head  end  and  exhausting  from  the  crank  end. 

Between  the  positions  of  Fig.  9  and  Fig.  10  both  ports  are 
entirely  closed,  expansion  is  taking  place  in  the  head  end  and  com- 
pression in  the  crank  end.  Fig.  10  is  head-end  release.  Fig.  11 
shows  admission  at  crank  end  of  cylinder  and  marks  the  end  of 
crank-end  compression. 


10 


VALVE  GEARS 


By  referring  to  Figs.  6-11,  the  effect  of  any  change  of  lap 
may  at  once  be  observed.  If  the  outside  lap  is  increased,  the 
valve  must  move  farther  from  mid-position  before  admission  will 
occur,  and  on  the  return,  after  the  maximum  displacement  is 
reached,  the  outside  lap,  being  wider,  will  close  the  port  sooner, 
and  the  cut-off  shown  in  Fig.  8  will  take  place  before  the  crank 


Fig.  10. 

reaches  the  angle  there  shown.     A  decrease  of  outside  lap  will 
make  cut-off  later  and  admission  earlier. 

If  the  inside  lap  is  increased,  the  valve  must  move  farther  be- 
fore release  occurs  and  the  crank  angle  would  be  greater  than  shown 
in  Fig.  10.  On  the  return  of  the  valve  to  the  dotted  position  shown 
in  Fig.  10,  the  port  will  close  earlier  and  make  an  earlier  compres- 
sion; the  crank  angle  will  be  less  than  is  there  shown.  Decreasing 
inside  lap  will  cause  earlier  release  and  later  compression. 


Fig.  11. 

Thus  we  see  that  it  is  the  outside  lap  that  influences  admis- 
sion and  cut-off,  and  the  inside  lap  that  controls  release  and 
compression.  For  this  reason  the  outside  lap  is  often  called  the 
steam  lap,  and  the  inside  lap  the  exhaust  lap. 


VALVE  GEARS 


11 


Fig.  12. 


Lead.  If  a  valve  having  lap  is  in  mid-position,  the  port  is 
closed  and  the  engine  cannot  start  because  no  steam  can  enter  the 
cylinder.  That  the  steam  may  be  ready  to  enter  the  cylinder  at 
the  beginning  of  the  stroke  it  is  necessary  that  the  eccentric  be 
set  more  than  90°  ahead  of  the  crank  and  the  eccentric  radius  will 
take  an  angle  as  shown  in  Fig.  6,  called  the  angular  advance.  In 
order  that  the  ports  and 
clearance  may  be  prop- 
erly filled  with  steam 
at  the  beginning  of  the 
stroke,  it  is  necessary 
that  the  valve  be  dis- 
placed from  its  mid- 
position  an  amount 
slightly  greater  than 
the  outside  lap.  With 
the  piston  at  the  end  of  the  stroke  the  valve  will  have  a  position  as 
shown  in  Fig.  12.  The  port  will  be  open  the  distance  AB.  This 
causes  the  eccentric  to  be  moved  forward  a  slight  amount  in  excess 
of  the  angular  advance.  This  excess  is  called  the  angle  of  lead. 

In  Fig.  13,  O'R'  represents 
the  crank  at  the  beginning  of  the 
stroke,  LOA  the  angular  ad- 
vance, and  ADA'  the  angle  of 
lead.  The  eccentric,  to  give 
lead,  must  be  set  at  the  angle 
EGA'  ahead  of  the  crank  or  90° 
plus  angular  advance  plus  angle 
of  lead.  In  large,  quick-run- 
ning engines,  a  liberal  lead  is 
essential,  so  that  the  ports  and 
clearance  may  be  well  filled 
with  steam  before  the  stroke 
begins.  If  there  is  no  lead,  a 


Fig.  13. 


portion  of  the  steam  will  be  used 
in  filling  these  places  and  full 
pressure  steam  will  not  reach  the  piston  until  it  is  well  advanced 
on  the  stroke.  This  will  give  a  sloping  admission  line  as 


12 


VALVE  GEARS 


in  Fig,  14.     Too  much  lead,  on  the  other  hand,  will  cause  too 
early  an  admission  as  shown  in  Fig  15. 

If  the  angular  advance  is  increased,  the  eccentric  will  be 
moved  farther  ahead  of  the  crank,  and  consequently  will  begin  its 
motion  sooner.  It  will  necessarily  arrive  at  each  of  the  events 


Fig  14.  Fig.  15. 

sooner  than  before.     If  then,  the  angular  advance  is  increased,  all 
of  the  events  of  the  stroke  will  occur  earlier. 

Inequality  of   Steam    Distribution.     In  the  valve  diagrams 
thus  far  considered,  the  events  of  the  stroke  have  been  discussed 
for  each  end  separately,  without  reference  to  the  relation  of  sim- 
ilar events  on  the  other  side  of  the 
piston.     If    the   connecting    rod 
were  of  infinite  length,  so  that  it 
would  always  remain  parallel  to 
the  center  line  of  the  engine,  the 
distribution   would   be  the  same 
-Y    for  both  ends  of  the  cylinder.    In 
practice,   the   connecting   rod    is 
from  4  to  8  times  tLo  length  of 
the  crank,  which  causes  the  con- 
necting rod  always  to  be  at  an 
angle  to  the   center   line  of   the 
engine,    and    for   a    given   crank 
angle  makes  the  piston  displace- 
ment greater  at  the  head  end  than  at  the  crank  end. 

To  find  the  displacement  of  the  valve,  let  us  consider  Fig.  16. 
The  circle  represents  the  path  of  the  eccentric  center  during  a 
complete  revolution  of  the  engine.  OC  represents  the  crank,  and 
OR  the  corresponding  position  of  the  eccentric.  The  diameter 
XY  represents  the  extent  of  the  valve  travel.  Since  the  eccentric 
rod  is  so  long  in  comparison  to  the  eccentricity,  we  make  no 
appreciable  error  by  assuming  it  always  to  be  parallel  to  the  center 


Fig.  16. 


VALVE  GEARS 


13 


line  of  the  engine.  When  the  eccentric  is  at  OL,  the  valve  is  in  mid- 
position.  At  OR  the  valve  has  moved  from  mid-position  an  amount 
ON,  found  by  dropping  a  perpendicular  from  R  to  the  center  line 
XY.  If  the  angularity  of  the  connecting  rod  could  be  neglected, 
't\\Q  piston  displacement  could  be  found  in  the  same  manner. 

To  find  the  displacement  of  the  piston,  a  diagram  as  shown 
in  Fig.  17  must  be  drawn.  In  this  figure  AB  represents  the 
cylinder,  P  the  piston,  II  the  crosshead,  HR  the  connecting  rod, 
and  OR  the  crank.  Suppose  now  the  engine  should  stop  in-  this 
position  and  then  be  clamped.  The  piston  displacement  would 
be  represented  by  AP.  If  the  crank  pin  at  R  should  now  be 
loosened  so  as  to  allow  the  connecting  rod  to  fall  to  a  horizontal  posi- 
tion, the  point  R  would  describe  the  arc  of  a  circle  RN,  and  XN 
would  represent  the  piston  displacement  and  would  be  equal  to  AP 


P' 


Fig.  17 


Suppose  now  that  in  this  disconnected  way  the  piston,  crosshead 
and  connecting  rod  were  moved  forward  until  the  end  of  the  rod 
came  to  O.  P  would  then  be  at  P'  and  the  piston  would  be  in  the 
middle  of  its  stroke.  Now  swing  the  end  of  the  rod  lip  to  its 
proper  position  on  the  crank-pin  circle,  the  piston  remaining  sta- 
tionary. It  would  describe  an  arc  OZ.  The  crank  pin  would  be 
at  Z,  less  than  a  quarter  revolution  from  X,  while  the  piston  would 
be  in  the  middle  of  its  stroke. 

Suppose  this  engine  were  running  with  cut-off  at  half  stroke 
on  the  head  end  and  that  XOZ  represented  the  corresponding  crank 
angle.  On  the  return  stroke  the  valve  .would  cut  off  at  the  same 
crank  angle  YOT  =  XOZ,  and  OT  would  represent  the  crank  cut- 
off on  the  return  or  crank-end  stroke.  The  piston,  as  we  have  just 
seen,  will  not  be  at  half  stroke  except  when  the  crank  is  at  OZ  or  OS. 
Consequently  OT  is  less  than  half  stroke  and  cut-off  takes  place 
earlier  at  the  crank  end  than  at  the  head  end.  When  the  crank  is  at 
OZ  the  eccentric  will  be  at  OA  (Fig  17#),  and  the  valve  displace- 


VALVE  GEARS 


merit  will  be  OB.  When  the  crank  is  at  OT  the  eccentric  will  be  at 
OA',  and  the  valve  displacement  will  be  OB',  which  is  equal  to  OB, 
the  displacement  of  the  valve  at  cut-off  in  the  head  end.  The  pis- 
ton displacement  will  be  OX  in  the  head  end  and  WY  in  the  crank 
end  when  cut-off  occurs.  If  the  connecting  rod  always  remained 
parallel  to  the  center  line,  the  cut-off  would  be  the  same  at  both  ends. 
Compensation  of  Cut=off.  It  has  already  been  pointed  out 
that  lengthening  the  outside  lap  makes  the  cut-off  earlier,  and  short- 
ening  the  lap  makes  it  later.  The  cut-off  in  the  case  just  cited  may 
then  be  equalized  by  altering  the  outside. laps.  If  we  increase  the 
outside  lap  on  the  head  end,  or  decrease  the  crank-end  lap,  the 
inequality  will  be  less.  By  changing  either  or  hoth  of  the  laps  the 
proper  amount,  the  cut-off  may  be  exactly  equalized. 


Fig.  11  a. 


But  altering  the  outside  lap  changes  the  lead  as  has  already 
been  explained.  If  the  lap  is  increased  on  the  head  end,  the  lead 
will  be  less  than  on  the  crank  end.  If  the  lead  becomes  too  small 
on  the  head  end,  the  angular  advance  may  be  increased  but  the 
inequality  of  lead  will  still  remain,  for  this  increase  of  angular 
advance  will  increase  the  lead  at  the  crank  end  as  well  as  at  the 
head  end,  and  by< hastening  all  the  events  of  the  stroke  may  give  a 
bad  steam  distribution  if  care  is  not  taken. 

Unequal  lead  is  of  less  consequence  on  a  low-speed  than  on  a 
high-speed  engine.  On  low-speed  engines  the  cut-off  may  be 
equalized  at  the  expense  of  lead  with  beneficial  results,  but  on  high- 
speed engines  it  will  not  do  to  give  too  little  lead  at  one  end.  A 
high-speed  engine  requires  more  lead  than  a  low-speed,  for  there  is 
relatively  less  time  in  each  stroke  for  the  clearance  to  fill  with  steam. 


VALVE  GEARS 


15 


If  both  inside  laps  are  equal,  compression  will  not  occur 
equally  at  both  ends.  To  equalize  it,  the.  inside  laps  may  be 
changed  in  the  same  manner  as  the  outside  laps  are  changed  to 
equalize  the  .cut-off.  By  altering  these  inside  laps  to  equalize 
compression,  it  may  happen  that  the  lap  is  reduced  enough  to 
leave  the  exhaust  port  open  when  the  valve  is  in  mid-position. 


Fig.  18. 

This  opening  of  the  valve  is  called  an  inside  clearance,  or  negative 
lap.     In  Fig.  18,  A  is  the  inside  clearance.  , 

Rocker.  Sometimes  it  happens  that  the  valve  stem  and 
eccentric  rod  cannot  be  so  placed  that  they  will  be  in  the  same 
straight  line;  or  it  may  be  that  the  travel  of  the  valve  must  be  so 
great  as  to  require  an  excessively  large  eccentric.  In  such  cases 
a  rocker  may  be  used. 

Fig;.  19  shows  a  valve  that  is  not  in  line  with  the  eccentric. 

O 

This  occurs  in  horizontal  engines  when  the  valve  is  set  on  top  of 


Fig.  19. 

the  cylinder  instead  of  on  one  side.     By  means  of  the  rocker  AG 
the  valve  may  receive  its  proper  motion. 

In  case  it  is  more  convenient  to  place  the  pivot  of  the  rocker 
arm  between  the  connections  to  the  valve  stem  and  those  of  the 
eccentric  rod,  such  an  arrangement  as  shown  in  Fig.  20  may  be 
used.  Here  it  will  be  noticed  that  the  valve  stem  and  eccentric 
rod  are  moving  in  opposite  directions,  and  to  give  the  valve  the 


16  VALVE  GEARS 


same  motion  as  in  Fig.  19,  the  eccentric  must  be  moved  180°  ahead 
of  the  position  there  shown. 

If  AB  is  less  than  AG,  the  valve  travel  will  be  greater  than 
twice  the  eccentricity,  in  proportion  as  AG  is  greater  than  AB. 
In  all  cases  the  valve  travel  is  to  twice  the  eccentricity  as  AG  is  to 
AB.  Thus,  if  the  valve  travel  is  4|  inches,  AB,  15  inches,  and 

AG,  18  inches,  then   —  X  4J  =  3|  inches,  will  equal  twice  the 
lo 

eccentricity. 


/ 


Fig.  20. 

A  valve  gear  may  be  so  laid  out  as  to  make  both  the  cut-off 
and  the  lead  equal  for  both  ends  of  the  cylinder.  This  may  be 
done  by  a  proper  porportion  between  the  rocker  arms,  and  a  careful 
location  of  the  pivot  of  the  rocker.  The  eccentric  must  then  be 
set  accordingly.  In  this  manner  the  Straight  Line  engine  equal- 
izes  the  cut-off  and  lead.  A  discussion  of  this  method  will  be 
considered  later. 

VALVE  DIAGRAMS. 

Zeuner'S'  Diagram.  In  order  to  study  the  movements  of 
valves,  the  effect  of  lap,  lead,  eccentricity,  etc.,  diagrams  of  various 
sorts  have  been  devised.  By  the  use  of  diagrams  we  may  acquire 
a  knowledge  of  valve  motion  without  the  complex  mathematical 
expressions  that  such  a  discussion  would  entail.  The  most  useful 
of  these  various  diagrams  is  that  devised  by  Zeuner,  and  to  avoid 
complexity  we  shall  confine  ourselves  to  a  discussion  of  this  dia- 
gram alone.  The  eccentric  rod  is  assumed  to  be  of  infinite 
length,  and  the  positions  of  the  crank  are  shown  on  the  diagrams. 
The  displacement  of  the  piston  can  easily  be  found  if  the  ratio  of 
crank  to  connecting  rod  is  known. 

In  Fig.  21  let  OY  be  the  eccentricity,  then  XOY  will  rep- 
resent the  valve  travel,  and  the  center  of  the  eccentric  will  move 


VALVE  GEARS 


17 


in  the  circle  XWY.  Let  OK  represent  the  position  of  the 
crank  and  Or  the  corresponding  position  of  the  eccentric,  which 
is  90°  -f-  angle  of  advance  6  ahead  of  the  crank.  Draw  OW 
perpendicular  to  XY  and  lay  off  from  it  the  angle  WOM  = 
angle  of  advance  6  towards  the  crank.  With  OH  as  a  diameter, 
construct  a  circle.  CM  is  equal  to  the  eccentricity,  and  the  circle 
MFC  is  known  as  the  valve  circle.  If  Oil,  the  center  line  of 


x  s 


the  crank,  cuts  this  valve  circle  at  P,  then  OP  is  equal  to  the  dis- 
placement of  the  valve  from  mid-position. 

To  prove  this,  draw  rS  perpendicular  to  XY.  Since  Or 
is  the  position  of  the  eccentric,  OS  will  represent  the  valve  dis- 
placement from  mid-position.  Draw  MP.  Then  by  geometry 
OPM  is  a  right  angle  because  it  is  inscribed  in  a  semicircle. 
OSr  is  also  a  right  angle  ;  the  two  right-angled  triangles  OSr 
and  OMP  are  equal  because  they  are  similar  and  have  two  cor- 
responding sides  equal.  Or  =  OM,  being  radii  of  the  same 
circle.  But  we  have  seen  that  OS  is  equal  to  the  valve  displace- 
ment, therefore  OP  is  also  equal  to  the  valve  displacement,  for  it 
is  equal  to  OS. 

Now  that  the  truth  of  our  proposition  has  been  proved,  let 
us  see  how  we  may  study  the  valve  motion  from  such  a  diagram. 
See  Fig.  22.  As  before,  let  XY  represent  the  valve  travel,  then 
the  circle  XEYF  will  represent  the  path  of  the  center  of  the 
eccentric.  Let  6  be  the  angular  advance  and  lay  off  EO  toward  the 
crank,  making  an  angle  0  with  the  vertical.  Produce  EO  to  F, 
and  on  OE  and  OF  as  diameters  draw  the  valve  circles  as  shown. 
Let  the  outside  lap.be  an  amount  equal  to  OY,  then  with  O  as  a 
center  and  OY  as  a  radius  draw  an  arc  intersecting  the  upper 
valve  circle  at  Y  and  K.  Lay  off  OP  equal  to  the  inside  lap  and 


18 


VALVE  GEARS 


with  O  as  center  and  OP  as  a  radius  draw  an  arc  intersecting  the 
valve  circle  at  P  and  Q.  Draw  the  crank  line  AO  passing  through 
Y.  Then,  when  the  crank  is  in  this  position,  the  displacement  of 
the  valve  is  equal  to  OY  (the  outside  lap)  and  the  steam  is  ready 
to  enter  the  cylinder.  This  is  the  position  of  the  crank  at  admis- 
sion, and  the  crank  angle  XOA  is  called  the  lead  angle.  The 
valve  has  lead,  therefore  the  admission  takes  place  before  the  end 
of  the  stroke.  When  the  crank  reaches  the  position  OE,  the 
displacement  of  the  valve  is  equal  to  OE,  the  eccentricity,  and  is 


90°*  e 


Fig.  22. 

the  maximum  displacement.  Further  motion  of  the  piston  causes 
the  valve  to  move  toward  mid-position  until,  at  the  crank  position 
OC,  the  displacement  OK  is  again  equal  to  the  outside  lap  and 
the  valve  has  reached  the  point  of  cut-off.  When  the  position  Oil 
is  reached,  the  crank  line  is  tangent  to  both  valve  circles  and  there 
is  no  displacement  of  the  valve.  At  this  point  the  valve  is  in 
mid-position. 

Further  crank  movement  draws  the  inside  lap  toward  the 
edge  of  the  exhaust  port  until,  at  the  crank  position  OB,  the  dis- 
placement is  equal  to  OP  (the  inside  lap)  s-nd  release  begins.  At 
OF  the  maximum  valve  displacement  is  again  reached  -and  the 
valve  moves  in  the  opposite  direction  until  at  OD  its  displacement 


VALVE  GEARS 


19 


from  mid-position,  is  again  equal  to  OQ  =  OP  =  the  inside  lap, 
and  compression  takes  place.  At  OH'  the  valve  is  again  in  mid- 
position.  At  OX  the  displacement  of  the  valve  is  OI,  but  since 
the  valve  has  to  move  the  distance  OJ  before  the  port  begins  to 
open  IJ  must  represent  the  port  opening  when  the  crank  is  on 
dead  center  and  by  definition  we  know  that  lead  is  the  amount  of 
port  opening  at  this  position.  Therefore  IJ  represents  the  lead. 

At  the  position  It,  the  port  is  open  an  amount  equal  to  TG, 
at  E  the  opening  is  a  maximum  equal  to  EN.  At  C  the  opening 
is  nothing.  If  LW  represents  the  total  width  of  the  steam  port, 
the  exhaust  port  will  be  open  wide  when  the  displacement  of  the 
valve  is  equal  to  OWand  it  will  remain  wide  open  while  the  crank 
swings  from  OW  to  OK. 


Fig.  23. 

If  the  width  of  steam  port  in  addition  to  the  outside  lap  were 
laid  off  on  the  other  valve  circle  it  would  fall  at  E'.  For  the  ad- 
mission port  to  be  wide  open,  the  displacement  of  the  valve  would 
have  to  be  equal  to  OE',' which  is  more  than  the  maximum  dis- 
placement. This  shows  that  in  this  case  the  steam  port  is  never 
fully  open  and  that  the  left-hand  edge  of  the  valve  overlaps  the 
right-hand  edge  of  the  port  by  an  amount  equal  to  EE'  when  the 
valve  has  reached  its  maximum  displacement. 


20- 


VALVE  GEARS 


Fig.  22,  with  its  two  valve  circles,  shows  the  diagram  for  the 
head  end  of  the  cylinder  only.  The  crank-end  diagram  would  be 
similar  except  that  the  laps  might  not  be  equal  to  those  of  the 
head  end. 

We  are  now  in  a  position  to  consider  more  in  detail  the  effect 
of  changing  in  any  wray  either  the  valve  or  the  setting.  Let  us  con- 
sider Fig.  23,  which  is  in  every  way  like  Fig.  22  except  that  all 
unnecessary  letters  and  lines  are  omitted  to  avoid  confusion.  If 
the  outside  lap  is  increased  an  amount  equal  to  !NM,  the  ad- 
mission will  take  place  later,  at  crank  position  OA';  the  lead  will 


Fig.  24. 

be  reduced  to  IG  and  cut-off  will  take  place  earlier  at  OC'.  If 
the  outside  lap  is  reduced  a  like  amount  the  contrary  effects  will 
be  observed.  If  the  inside  lap  is  increased  an  amount  equal  to  LS. 
the  release  will  take  place  later  at  the  crank  position  OB'  and  com- 
pression will  take  place  earlier  at  OD'.  The  contrary  effect  will 
be  observed  by  decreasing  the  inside  lap. 

If  the  angular  advance  is  increased  (see  Fig.  24)  all  the  events 
will  occur  earlier.  This  is  evident  from  the  figure;  the  crank 
revolves  in  the  direction  indicated  by  the  arrow  and  OA'  (new  posi- 
tion of  admission)  is  ahead  of  OA,  the  old  position. 


VALVE  GEARS 


21 


If  the  eccentricity  is  increased,  Fig.  25,  the  valve  travel  will 
increase  and  admission  will  take  place  earlier  at  OA';  the  lead 
will  be  increased  an  amount  equal  to  II',  and  cut-off  will  take 
place  later  at  OC'.  Release  will  be  earlier  at  OB'  and  com- 

C1 


Fig.  25. 

pression  will  be  later  at  CD'.  The  upper  valve  circle  will  now 
cut  the  arc  drawn  from  O  as  a  center,  with  a  radius  equal  to  the 
outside  lap  plus  the  width  of  steam  port,  in  the  points  W  and  H', 
and  the  admission  port  will  be  open  wide  while  the  crank  is  mov- 
ing from  OW  to  OH'.  Similarly,  the  lower  valve  circle  cuts  the 
arc  drawn  from  O  as  a  center,  with  a  radius  equal  to  the  inside 
lap  plus  the  width  of  steam  port,  in  the  points  W  and  H.  The 
steam  port  is  then  wide  open  to  exhaust  while  the  crank  is  moving 
from  W  to  H.  From  the  above  it  will  be  seen  that  the  periods 
are  all  changed  by  changing  the  travel;  thus,  admission  and  ex- 
haust begin  sooner  and  last  longer,  while  expansion  and  com- 
pression begin  later  and  cease  sooner.  With  change  in  the  angular 
advance,  however  (see  Fig.  24),  the  periods  are  neither  increased 
nor  decreased. 


22 


VALVE  GEAKS 


For  convenience,  these  results  are  collected  in  the  following 
table  which  shows  the  effect  of  changing  the  laps,  travel,  and 
angular  advance: 


Increasing 
Outside  Lap. 

Increasing 
Inside  Lap. 

Increasing 
Travel. 

Increasing 
Angular  Advance. 

Admission. 

Is  later. 
Ceases  sooner. 

Not  changed. 

Begins  earlier. 
Continues 
longer. 

Begins  earlier 
Same  period. 

Expansion. 

Is  earlier. 
Continues 
longer. 

Beginning 
unchanged. 
Continues 
longer. 

Begins  later. 
Ceases  sooner. 

Begins  earlier. 
Same  period. 

Exhaust. 

Unchanged. 

Occurs  later. 
Ceases  sooner. 

Begins  earlier. 
Ceases  later. 

Begins  earlier. 
Same  period. 

Compression 

Begins  at  same 
point. 

Begins  sooner. 
Continues 
longer. 

Begins  later. 
Ceases  sooner. 

Begins  earlier. 
Same  period. 

PROBLEMS. 

All  the  problems  on  valve  gears  involve  the  relations  between 
certain  variables  which  are  : 

The  valve  travel. 
Angle  of  lead. 
Outside  lap. 
Inside  lap. 

Points  of  stroke  at  which  admission  cut-off,  release  and  compression 
take  place. 

In  designing  a  Slide  Valve,  a  few  of  these  variables  depend 
upon  the  conditions  under  which  the  engine  is  to  run.  For  instance, 
the  valve  travel  is  limited,  cut-off  must  be  at  a  certain  point  and 
the  engine  must  have  a  certain  lead.  Then,  with  the  aid  of  a  Zeun- 
er's  diagram,  the  remaining  proportions  of  the  valve  may  be  deter- 
mined. 

Let  us  consider  a  few  examples: 

Given  the  valve  travel  =  3  inches. 
Inside  lap  =  %  inch. 

Angular  advance  =  35° 

Angle  at  cut-off  =  115° 


VALVE  GEARS 


23 


To  determine  the  laps,  the  lead  and  the  crank  angles  at 
admission,  compression  and  release. 

In  Fig.  26,  let  XY  represent  the  valve  travel  =  3  inches. 
Draw  OM  perpendicular  to  XY,  and  on  XY  as  a  diameter  draw 
the  circle  XMYF  representing  the  path  of  the  center  of  the  eccen- 
tric as  it  revolves  about  the  shaft.  Lay  off  the  angle  MOE  —  the 
angular  advance  =  35°  so  that  the  angle  XOE  is  equal  to  90° 


Fig.  26. 

minus  the  angular  advance0  Produce  EO  to  F.  Then  on  OE 
and  OF  as  diameters  draw  the  valve  circles.  The  eccentricity 
OE  or  OF,  if  no  rocker  is  used,  will  be  half  the  valve  travel.  Lay 
off  the  crank  angle  XOC  =  angle  of  crank  at  cut-off  =  115°,  and 
OK  will  then  represent  the  distance  of  the  valve  from  mid-posi- 
tion when  cut-off  takes  place.  This  distance  we  know  is  the  out- 
side lap.  Draw  the  arc  KI,  known  as  the  lap  circle,  and  it  will 
cut  the  valve  circle  again  at  V.  When  the  valve  is  again  the  dis- 
tance OV  =  the  outside  lap  from  mid-position,  admission  will  take 
place.  Draw  the  line  OVA  and  this  will  represent  the  position 
of  the  crank  at  admission. 


24  VALVE  GEARS 


When  the  crank  is  at  OX,  the  valve  displacement  is  equal  to 
OJ.  This  is  at  dead  center  and  the  valve  is  open  the  amount  IJ, 
for  it  has  moved  this  distance  more  than  the  outside  lap.  There- 
fore IJ  is  the  lead  for  this  end. 

Now  on  the  other  valve  circle,  draw  the  arc  PQ  with  the 
inside  lap  (|-  inch)  as  a  radius.  It  will  cut  the  valve  circle  at 
P  and  Q.  When  the  valve  displacement  is  equal  to  OQ,  the 
exhaust  port  has  just  closed  and  the  engine  is  at  compression.  In 
the  same  way  OP  is  the  valve  displacement  at  release  when  the 
port  begins  to  open.  OQD  represents  the  crank  position  at  com- 
pression and  OPB  the  crank  position  at  release. 

The  results  then  are  as  follows  : 

Given : 

Valve  travel  =  XY  =  3  inches. 

Angular  advance          =  angle  MOE  =  35°. 
Inside  lap  =  OP  =  %  inch. 

Crank  angle  at  cut-off  =  angle  XOC  115° 
Found : 

Outside  lap  =  OK      =  %  inch. 

Angle  of  lead  =  XOA  =  5°. 

Linear  lead  =  IJ        =  ¥8¥  inch. 

Max.  port  opening  for  admission  =  HE     =  %  inch. 
Crank  angle  at  compression         =  XOD  =  185° 
Crank  angle  at  release  =  XOB  =  65° 

Max.  port  opening  for  exhaust    =  FN      =  %  inch. 

Fig.  26  is  drawn  full  size,  and  all  of  these  measurements  may 
readily  be  verified.  This  figure  is  drawn  for  the  head  end  only. 
If  the  crank  angle  at  cut-off  is  the  same  on  both  ends,  the  Zeuner's 
diagram  for  the  crank  end  will  be  exactly  like  Fig.  26. 

ANOTHER  PROBLEM. 
Given : 

The  valve  travel  =  3  inches. 

The  lead  angle  =  6°. 

Crank  angle  at  cut-off  =  70°. 

Crank  angle  at  compression  =  75°. 
To  Find : 

Angular  advance. 

Laps. 

Linear  lead. 

Crank  angle  at  release. 

As  before,  let  XY  represent  the  valve  travel  =  3  inches  and 
draw  OM  and  the  circle  XMYF.  See  Fig.  27.  Lay  off  the  lead 


VALVE  GEARS 


angle  XOA  =  6°.  Then  OA  represents  the  crank  position  at 
admission.  Next  lay  off  the  crank  angle  XOC,  the  angle  at  cut- 
off 70°.  Bisect  the  angle  COA  by  the  line  OE  and  on  OE  draw 
the  valve  circle.  Angle  MOE  =  the  angular  advance.  The  valve 
circle  will  cut  the  crank  lines  OC  and  OA  at  Kand  V  respectively. 
If  the  work  has  been  carefully  done,  OK  will  be  exactly  equal  to 
OY  and  will  represent  the  outside  lap.  The  lead  is  IJ  as  before. 
Draw  OD  at  position  of  the  crank  at  compression  so  that  angle 
XOD  =  75°.  Continue  OE  to  cut  the  eccentric  circle  at  F.  On 


B 


Fig.  27. 

OF  draw  the  second  valve  circle.  It  will  cut  OD  at  Q,  and  OQ 
will  represent  the  inside  lap.  Draw  the  lap  circle  OP,  and  the 
crank  position  OPE.  This  will  be  the  crank  position  at  release. 

The  angular  advance  in  this  problem  is  large  and  all  the 
events  of  the  stroke  are  early.  Compression  and  release  are  excess- 
ively early  and  the  outside  lap  is  unusually  large.  In  the  previ- 
ous problem,  with  cut-off  at  about  two-thirds  stroke,  the  results 
were  nearly  normal.  Cut-off  with  the  plain  slide  valve,  earlier 
than  half  stroke  cannot  be  had  without  sacrificing  the  steam  dis- 
tribution on  the  other  events. 


26  VALVE  GEAKS 


To  sum  up  we  have 

liven : 

Valve  travel  =  XY  —  3  inches. 

Lead  angle  =  XOA  =  6°. 

Crank  angle  at  cut-off  =  XOO  =  70°. 

Crank  angle  at  compression  =  XOD  =  75°. 

To  find : 

Angular  advance  —  MOE  —  58°. 

Outside  lap  =  OV      —  1  &  inches. 

Lead  =  I J        =  &  inch. 

Inside  lap  =  OQ      =  37a  inch. 

Orank  angle  at  release  =  XOB  =  ISO3. 

Suppose  in  this  last  problem  the  cut-off  had  been  given  at 
half  stroke  instead  of  having  the  crank  angle  given,  and  that  the 
compression  had  been  given  in  the  same  way.  AVe  should,  of 
course,  need  to  know  the  ratio  of  length  of  connecting  rod  to 
crank.  Let  this  be  given  as  4,  that  is,  the  connecting  rod  is  four 
times  the  length  of  the  crank. 

In  Fig.  28  let  XY  represent  the  valve  travel.  Extend  XY 
to  the  left  to  the  point  Z,  and  make  OZ  equal  to  four  times  OX. 
With  Z  as  a  center  and  OZ  as  a  radius,  strike  an  arc  OC  that  will 
cut  the  eccentric  circle  at  C;  then  draw  OC,  which  will  represent 
the  crank  when  the  piston  is  at  half-stroke,  which  is  assumed  to 
be  the  point  of  cut-off. 

To  find  the  crank  angle  at  compression,  lay  off  YII  equal 
to  .8  of  the  distance  YX.  From  II  lay  off  II W  =  OZ  =  four  times 
OX.  From  "VV  as  a  center  with  a  radius  "WTI,  draw  an  arc  cut- 
ting the  eccentric  circle  at  D.  Draw  OD,  which  will  represent 
the  position  of  the  crank  at  compression. 

The  student  is  advised  to  read  over  again  pages  13  to  14  if  this  expla- 
nation of  finding  the  crank  angle  does  not  seem  perfectly  clear. 

ANOTHER  PROBLEM, 
Given : 

Cut  off  at  .6  stroke. 

Lead  =  fa  inch. 

Maximum  port  opening  —  %  inch, 

Katio  of  crank  to  connecting  rod  -  4. 

k          To  find : 

The  eccentricity. 
Lead  angle. 
Angular  advance. 
Laps. 


VALVE  GEARS 


In  Fig.  29  assume  an  eccentricity  that  will  if  possible  be  a 
little  too  large.  Let  us  take  for  trial  2J  inches  and  draw  XY 
equal  to  twice  the  assumed  eccentricity  equal  to  4^  inches.  Lay 
off  XC'  equal  to  .0  of  XY, 
and  with  a  radius  equal  to 
four  times  OX  draw  the  arc 
C'C  as  already  explained. 

Then  draw  OC,  which 
will  represent  the  position  of 
the  crank  at  cut-off,  and 
XOCwill  be  the  crank-angle 

o 

at  cut-off.  Assume  a  lead 
angle  of  about  7°  and  draw 
OA,  which,  if  this  assump- 
tion be  true,  will  represent 
the  crank -angle  at  ad  miss  ion. 

G> 

Bisect  the  angle  CO  A  by  the 
line  OE,  and  on  OE  draw 
the  valve  circle.  Draw  the 
.lap  circle  VNK.  With  this 
assumed  eccentricity  we  find 
a  maximum  port  opening  of 
NE  =  .75  inch,  which  is 
larger  than  the  conditions  of 
the  problem  demand.  We 
may  then  form  a  proportion, 
namely: 

The  actual  port  opening  de- 
sired :  the  port  opening  with  the 
assumed  eccentricity  : : probable 
eccentricity:  assumed  eccentric- 
ity. 

Substituting  the  figures 
we  have  .5  :  .75  :  :  x  :  2J  .-. 
x  —  the  probable  eccentric- 
ity; equals  1.42  inches. 

Now  draw  on  OE,a  new 
valve  circle  (dotted)  with  a  diameter  equal  to  the  required  eccen- 
tricity of  1.42  inches.     See  Fig.  29a.     It  will  cut  the  crank  line 


28 


VALVE  GEARS 


OC  at  K',  and  OK'  will  be  the  new  outside  lap  and  IT  will  be 
the  new  lead  (assuming  the  lead  angle  to  be  7°).  This  lead  I'J'  is 
^  inch,  while  the  required  lead  is  only  T^  inch.  Now  decrease  the 
angular  advance  enough  to  correct  one  half  of  this  difference,  by 
drawing  a  new  lap  circle  J"K"  of  -Jfl  inch  greater  radius.  This 
will  make  the  valve  circle  cut  OC  at  K",  so  that  OK"  will  now  be 


the  final  lap,  and  I"J';  the  final  lead,  which  is  equal  to  the  required 
j\  inch.  The  lead  angle  is  now  XO  A'  instead  of  XOA.  The  port 
opening  at  Nil'  is  -|  inch  (nearly)  as  required,  but  the  change  in 
angular  advance  necessitates  an  increase  of  lap  if  cut-off  is  to 
remain  the  same.  This  reduces  the  port  opening  by  the  amount 
HI!',  so  that  the  maximum  opening  is  only  .46  inch.  By  increas- 
ing the  eccentricity  this  port  opening  may  be  increased. 


VALVE  GEARS 


29 


s-i 
t'c 


.46  :  .50  :  :  1.36  :  x. 

x  =  1.48,  the  true  eccentricity. 

Now  draw  the  valve  circle  on  OE'  with  a  diameter  of  1.48 
inches.  It  will  cut  GO  in  lv'"  and  OX  in  J"'.  The  lap  will  be 
OK'"  =  .97  inch,  the  lead  will  be  T"  J'"  —  -^  inch,  the  angular 
advance  will  be  MOE'  and  the  eccentricity  ON'. 

To  sum  up  we  have 

Given : 

The  cut-off  =  .6  stroke. 

The  lead  =  -*g  inch 

Max.  port  opening  =  }•£  inch. 


30  VALVE  GEARS 


Obtained  all  of  the  above  conditions  together  with: 

Lap  =  .97  inch. 

Lead  angle  =  XOA'  ° 

Angular   advance   =  MOE'  ° 

Compression,  release  and  inside  laps  are  found  as  in  the  pre- 
vious problems. 

There  are  of  course  all  sorts  of  combinations  that  would  make 
up  different  problems,  but  they  can  all  be  solved  in  the  same  gen- 
eral way,  as  they  are  modifications  of  the  above. 

DESIGN  OF  THE  SLIDE  VALVE. 

In  designing  a  slide  valve  some  of  the  variables  are  assumed 
and  the  others  are  found  by  means  of  diagrams  as  we  have  already 
seen.  These  diagrams  show  only  the  dimensions  of  the  inside  and 
outside  laps  and  travel  of  valve;  the  other  dimensions  of  the  valve 
and  seat  must  be  calculated. 

Area  of  Steam  Pipe.  Pipes  that  supply  the  steam  chest 
should  be  large  enough  to  prevent  an  excessive  loss  of  pressure  due 
to  friction.  If  the  pipes  are  long  they  should  be  of  such  si^e  that 
the  mean  velocity  of  steam  in  them  does  not  exceed  100  feet  per 
second  or  6,000  feet  per  minute.  For  this  calculation  it  is.  usual 
to  assume  steam  admitted  to  the  cylinder  during  the  whole  stroke. 

For  example.  Suppose  an  engine  is  10"  X  18",  and  makes 
180  revolutions  per  minute.  What  is  the  diameter  of  the  steam, 
pipe? 

The  piston  displacement  or  volume  of  the  cylinder  is  : 

TTvZ2  3.1416    X   10' 

-  X   I  =  -  X    18  =  1413.  i  2  cubic  inches. 

4  4 

'   ^  =  .818  cubic  feet. 
IT/oo 

If  the  engine  makes  180  revolutions  it  would  use  2  X  180  X 
.818  =  294.48  cubic  feet  per  minute. 

294.48 


The  area  would  be  =      0'       :=  .04908  sq.  ft.  ==  7.0675  sq. 


in. 


The  diainoter  corresponding  to  7.0(575  square  inches  is  3 
inches. 

A  three- inch  pipe  would  be  large  enough,  especially  if  the 
engine  cut  off  early  in  the  stroke, 


VALVE  GEARS  31 


For  a  very  large  engine  cutting  off  early,  the  allowable  veloc- 
ity may  be  taken  as  8,000  feet  per  minute  instead  of  6,000  feet. 

Width  of  Steam  Port.  The  port  opening  at  admission  should 
give  nearly  as  great  an  area  as  the  steam  pipe  in  order  to  prevent 
loss  of  pressure  due  to  wire-drawing,  but  the  actual  width  of  the 
port  should  be  great  enough  for  the  free  exhaust  of  steam.  It 
is  well  to  have  the  steam  port  a  little  larger  than  the  area  of  the 
steam  pipe,  then  with  a  port  opening  of  .6  to  .9  of  the  port  area 
for  admission  and  full  port  opening  at  exhaust,  satisfactory  condi- 
tions will  result. 

The  length  of  the  ports  is  usually  made  about  .8  the  diameter  ol 
the  cylinder.  Then  in  the  10"  X  18"  engine  the  steam  ports  would 
be  8  inches  long.  If  the  area  for  admitting  steam  is  8.0675  squan 
inches  and  the  length  of  port  is  8  inches,  the  width  wTill  be 

=  _1— — -=  .8834  inch,  or  about  J  inch. 

The  width  of  port  opening  would  be  about  ,9  X  .8834  =  .79506 
inch  or  about  -if  inch. 

Width  of  Exhaust  Port.  When  the  slide  valve  is  at  its 
maximum  displacement,  the  valve  overlapping  the  exhaust  port 
as  shown  in  Fig.  7  reduces  the  area  more  or  less.  In  designing 
the  valve,  the  exhaust  port  should  be  of  such  a  width  that  the 
maximum  displacement  of  the  valve  does  not  reduce  the  area  of 
the  exhaust  port  to  less  than  the  area  of  the  steam  port.-  It  is  not 
advisable  to  make  the  exhaust  port  too  large  for  this  increases 
the  size  of  the  valve  and  thus  causes  excessive  friction. 

The  height  of  the  exhaust  cavity  should  never  be  less  than  the 
width  of  the  steam  port,  and  may  be  made  much  higher  to  advantage. 

Width  of  Bridge.  The  bridge  must  be  of  sufficient  width  so 
that  outside  edges  of  the  valve  cannot  uncover  the  exhaust  port. 
The  width  of  the  steam  port  plus  the  width  of  the  outside  lap  plus 
the  width  of  the  bridge  must  be  greater  than  the  maximum  dis- 
placement. 

The  width  of  the  bridges  should  be  not  less  than  the  thickness 
of  the  cylinder  wall  in  order  to. make  a  good  casting. 

The  Point  of  Cut=off.  In  the  study  of  Indicators,  it  was  shown 
that  if  the  point  of  cut-off  is  early,  the  other  events  are  not  good. 
If  a  plain  slide  valve  is  used  with  an  automatic  cut-off,  the  cut-off 


32  VALVE  GEARS 


is  hastened  either  by  changing  the  eccentricity  or  by  changing 
the  angular  advance.  Either  of  these  methods  will  accomplish  the 
result  at  the  expense  of  the  compression  which  consequently  will 
be  earlier  and  excessive.  Except  for  locomotives  and  high-speed 
engines,  where  compression  is  an  advantage,  the  plain  slide  valve 
is  not  arranged  to  cut-off  earlier  than  -J  or  §  stroke.  If  an  earlier 
cut-off  is  desired,  large  outside  laps  are  necessary.  The  cut-offs 
may  be  equalized  by  giving  the  head  end  a  greater  lap  than  the 
erank  end,  but  this  will  cause  an  inequality  of  lead. 

Lead.  The  lead  of  stationary  engines  varies  from  zero  to  | 
inch  according  to  the  style  of  engine.  An  engine  having  high 
compression  that  compresses  the  steam  nearly  to  boiler  pressure, 
will  give  good  results  with  little  or  no  lead.  If  the  ports  are  small, 
and  the  clearance  large,  there  should  be  considerable  lead  in  order 
to  insure  full  initial  pressure  on  the  piston  at  the  beginning  of  the 
stroke.  Valves  that  open  slowly  require  more  lead  than  quick- 
acting  valves. 

Let  us  design  and  lay  out  the  valve  and  valve  seat  for  the  fol- 
lowing engine: 

Diameter  of  cylinder  =  10  inches. 

Stroke  =  18  inches. 

Revolutions  =  180  per  minute. 

Lead  angle  =  3°. 

Cut-off  to  be  equal  at  both  ends  and  to  take  place  at  .75  stroke. 

Max.  port  opening  —  .9  area  of  steam  pipe. 

Compression  to  be  .85  of  the  stroke  at  both  ends. 

Length  of  connecting  rod  =  3  feet. 

The    'piston     displacement,    or    cylinder     volume,     will    be 

-  X 18  ==  1413.7  cubic  inches  or  .818  cubic  feet,     If  the 

engine  makes  180  revolutions,  it  will  use  2  X  180  X  .818  =  294.48 

cubic  feet  of  steam  per  minute.     Steam  pipe  area  =      ..          =  .0491 

square  feet  =  7.07  square  inches. 

This  7.07  square  inches  would  also  be  the  least  possible  area 
of  the  steam  ports.  If  the  length  of  port  is  made  .8  the  diam- 

7.07 
eter  of  cylinder,  the  width    will  be   -^—  =  .88  inches  or  about  J 

inch.  The  width  of  maximum  port  opening  will  be  .9  X  .88  =  .792 
or  nearly  j- j|  inch. 


VALVE  GEAKS 


33 


X 

A 


It  will  be  necessary  to  draw  a  separate  valve  circle  for  eacli 
end  of  the  cylinder.  First  consider  the  head  end. 

The  valve  travel  not  being  known,  we  shall  lay  off  XY  on  an 
assumption  of  6  inches  travel  and  draw  the  eccentric  circle  as 
shown  in  Fig.  30.  Lay  off  the  lead  angle  XOA  —  3°.  Lay  off 
XC'  =  .75  of  the  assumed  valve  travel  =  4J  inches.  Draw  the 
are  CC'  as  previously  explained  and  draw  OC  which  will  be  the 
crank  angle  at  cut-off.  The  radius  of  the  arc  C'C  will  be  equal  to 


G' 


.H 


V1 


V 


rA' 


1 


H 


Fig.  30. 

4  times  the  radius  of  the  eccentric  circle,  or  12  inches,  because 
the  connecting  rod  is  4  times  the  length  of  the  crank.  Bisect 
the  angle  AGO  by  the  line  ()E,  and  on  OE  draw  the  valve  circle. 
OV  =  OK  is  then  the  outside  lap,  with  these  assumed  condi- 


34 


VALVE   GEARS 


tions.  Draw  the  lap  circle;  then  EN  will  be  the  maximum  port 
opening.  EN  ==  1^  inches,  while  i-J  inch  is  all  that  is  necessary. 
The  assumed  eccentricity  is  3  inches,  therefore  the  probable  eccen- 
tricity =  x  :  3  :  :  i-J.  :  1T76.  x  =  1-J-j-  inches. 

Now  draw  a  new  eccentric  circle  with  a  radius  of  1-J-J  inches 
and  a  new  valve  circle  wTith  OE'  =  1-J-J  inches  as  a  diameter.  OK' 
is  now  the  outside  lap  and  the  maximum  port  opening  is  equal  to 
E'N',  which  from  actual  measurement  is  found  to  be  -}|-  inch.  The 
outside  lap  =  OK'  —  OY'  —  |~J  inch  and  the  lead  is  IJ  =  -f^  inch. 

Produce  EG  to  F  and  draw  another  valve  circle.  We  shall 
use  this  valve  circle  to  determine  the  outside  laps  and  lead  for  the 
crank  end  of  the. cylinder.  Since  the  cut-off  is  to  be  .75  of  the 
stroke,  we  may  lay  off  Oil'  —  OC',  and  with  a  radius  of  12  inches 


draw  the  arc  HIT.  Then,  as  already  explained,  OH  will  be  the 
crank  angle  at  cut-off  on  the  return  stroke.  OB  will  be  the  out- 

o 

side  lap  =  -if-  inch.  Draw  the  lap  circle  intersecting  the  valve 
circle  at  D.  Then  ODA'  is  the  crank  angle  at  admission  on  the 
return  stroke  and  LM  —  |  inch  is  the  lead  on  the  crank  end  of  the 
cylinder.  The  maximum  port  opening  will  always  be  greater  at  the 
crank  end  than  at  the  head  end  because  the  crank  end  lap  is  less 
in  order  to  get  the  equal  cut-off.  If  the  laps  were  equal,  -of  course 
the  port  openings  would  be  equal. 

Now  lay  off  YG'  =  .85  of  XY  and  find  the  crank  position 
OG.  This  is  the  compression  on  the  head  end  of  the  cylinder  and 
gives  an  inside  lap  on  this  end  of  ^\  inch,  which  is  equal  to  OP, 


VALVE  GEAKS 


Draw  the  lap  circle  PQ,  which  allows  us  to  draw  through  Q  the 
crank  line  Oil,  which  is  the  release  on  the  forward  stroke. 

Lay  off  XS'  =  :  YG'  =  .85  of  XY,  and  construct  the  crank 
line  OS,  which  is  the  crank  position  at  the  crank  end  compression. 
OS  intersects  the  valve. circle  at  T,  which  gives  OT  =  T?6  inch  = 
inside  lap  on  the  crank  end.  Draw  this  lap  circle,  which  will 
intersect  the  valve  circle  at  U.  This  enables  us  to  draw  OUW,  the 
crank  angle  at  release,  on  the  return  stroke. 

From  the  data  determined  by  means  of  these  diagrams  the 
valve  may  now  be  laid  out.  For  convenience  let  us  tabulate  the 
results  obtained  as  follows: 

Data.  Head  End.  Crank  End. 

Cut-off,  per  cent  of  Stroke  75  75 

Outside  Lap  |J"  -J-f" 

Inside  Lap  ?y  ^'' 

Lead  -g-V  I" 

Port  Opening  -L»"  lyV' 
Width  of  Port                                        -f  "  |" 

Fig.  31  shows  this  valve  in  section..  Let  us  begin  at  the  end 
having  the  largest  inside  lap,  or  in  this  case  at  the  crank  end. 
Layout  the  steam  port  |  inch  wide,  and  the  crank-end  outside  lap 
=  -if  inch.v'  The  bridge  will  be,  say,  |  inch  wide.  From  the 
inner  edge  of  the  steam  port,  lay  off  the  crank-end  inside  lap  = 
r76  inch.  When  the  valve  moves  to  the  left,  the  point  E'  will 
travel  1-J-J  inches,  a  distance  equal  to  the  eccentricity,  and  in  this 
position  of  extreme  displacement  the  exhaust  port  EF  must  be  open 
an  amount  at  least  equal  to  the  steam  port,  J  inch.  Therefore  we 
lay  off  EF  equal  to  1 J -  J-"  -f  |"  =  2T96".  The  inside  lap  overlaps  the 
bridge  nearly  -J  inch,  so  that  we  shall  have  to  make  the  exhaust 
port  opening  equal  to  2|  inches.  Lay  off  |  inch  again  for  the 
bridge  and  measure  back  ^V  inch,  equal  to  the  head-end  inside 
lap.  The  port  is  J  inch  wide,  and  the  head-end  inside  lap  of  |J 
inch  completes  the  outline  of  the  valve  seat. 


VALVE    SETTING. 

The  principles  of  valve  diagrams  are  useful  in  setting  valves 
as  well  as  in  desionino-  them.     The  valve  is  usually  set  as  accu- 

!~i  !— >  » 

rately  as  possible,  and  then,  after  indicator  cards  have  been  taken, 


36 


VALVE   GEARS 


the  final  adjustment  can  be  made  to  correct  slight  irregularities. 

The  slide  valve  is  so  designed  that  the  laps  cannot  be  altered 
without  considerable  labor,  and  the -radius  of  the  eccentric,  which 
determines  the  travel  of  the  valve,  is  usually  fixed.  The  adjust- 
able parts  are  commonly  the  length  of  the  valve  spindle  and  the 
angular  advance  of  the  eccentric. 

By  lengthening  or  shortening  the  valve  spindle,  the  valve  is 
made  to  travel  an  equal  distance  each  side  of  the  mid-position. 
Moving  the  eccentric  on  the  shaft  makes  the  action  of  the  valve 
earlier  or  later  as  the  angular  advance  is  increased  or  decreased. 

To  Put  the  Engine  on  the  Center.  It  is  usual  to  put  the 
engine  on  center  before  setting  the  valve.  First  put  the  engine 
in  a  position  where  the  piston  has  nearly  completed  the  outward 


RAM 

Fig.  32. 

Stroke,  and  make  a  mark  M  on  the  guide  opposite  the  corner  of 
the  crosshead  .or  at  some  convenient  place.  Also  make  a  mark, 
with  a  center  punch,  on  the  frame  of  the  engine  near  the  crank 
disc  or  on  the  floor.  With  this  punch  mark  Pas  a  center,  describe 
an  arc  C  on  the  wheel  rim,  v/itli  a  tram.  A  tram  is  a  steel  rod 
with  its  ends  bent  at  right  angles  and  sharpened. 

Turn  the  engine  past  the  center  until  the- mark  on  the  guide 
again  corresponds  with  the  corner  of  the  crosshead,  and  make 
another  mark  D  on  the  wheel  with  the  tram,  keeping  the  same 
center.  AVith  the  center  of  the  pulley  or  crank  disc  MS  a  center, 
describe  an  arc  CD  on  the  rim,  which  intersects  the  two  arcs  drawn 
with  the  tram.  Bisect  the  arc  CD  on  the  rim,  included  between 
the  two  short  arcs,  and  turn  the  engine  until  the  new  point  E  is  at 


VALVE  GEARS  37 


a  distance  from  the  point  on  the  frame  equal  to  the  length  of  the 
tram,  in  which  position  the  engine  will  be  on  the  center. 

The  engine  should  always  be  moved  in  the  direction  in  which 
it  is  to  run  so  that  the  lost  motion  of  the  wrist  pin  and  crank  pin 
will  be  taken  up  the  right  way.  In  case  the  engine  has  been 
moved  too  far  at  any  time,  it  should  be  turned  back  beyond  the 
desired  point  and  brought  up  to  that  point  while  the  engine  is 
moving  the  right  way. 

To  Set  the  Vaive  with  Equal  Lead.  Set  the  engine  on  the 
dead  point  and  give  the  eccentric  the  proper  angular  advance. 
Adjust  the  length  of  the  valve  spindle  to  give  the  proper  lead  for 
that  end.  Now  place  the  engine  on  the  other  dead  point  and 
measure  the  lead  at  that  end.  If  the  leads  are  unequal,  correct 
half  the  error  by  changing  the  length  of  the  valve  spindle  and 
the  other  half  by  altering  the  angular  advance.  In  case  the  valve 
gear  has  a  rocker,  the  length  of  the  spindle  should  be  such  that 
the  rocker  will  move  as  designed.  The  angular  advance  should 
not  be  changed,  but  the  equal  lead  should  be  obtained  by  means 
of  the  valve  spindle  or  the  eccentric  rod. 

Second  Method,  In  case  it  is  difficult  to  turn  an  engine  the 
following  method  may  be  used.  First  loosen  the  eccentric  on 
the  shaft  and  turn  it  around  until  it  gives  maximum  port  opening 
first  at  one  end  and  then  at  the  other.  If  the  maximum  port 
openings  are  not  equal,  make  them  so  by  changing  the  length  of 
the  valve  spindle  by  half  the  difference.  When  the  above  adjust- 
ment has  been  made,  set  the  engine  on  dead  center  and  give  the 
valve  the  proper  lead  by  turning  the  eccentric  on  the  shaft.  The 
angular  advance  is  thus  adjusted. 

To  Set  the  Valve  for  Equal  Cut=off.  Place  the  engine  on  the 
dead  point,  give  the  eccentric  the  proper  angular  advance  and 
the  valve  the  proper  lead.  Move  the  engine  forward  until  cut-off 
occurs,  then  measure  the  displacement  of  the  crosshead  from  the 
beginning  of  the  stroke.  Continue  moving  the  engine  forward, 
until  cut-off  takes  place  on  the  return  stroke  and  measure  the  dis- 
placement of  the  crosshead  from  the  beginning  of  this  stroke  to 
this  point. 

In  case  the  cut-off  is  earlier  at  the  crank  than  at  the  head-end, 
the  valve  spindle  is  too  short.  Adjust  the  length  of  the  spindle 


38  VALVE  GEARS 


so  that  the  inequality  will  be  corrected.  Now  set  the  engine  on 
the  dead  point  again  and  give  the  valve  the  proper  lead  by  means 
of  the  eccentric.  By  repeating  the  process,  making  slight  changes, 
the  desired  result  will  be  obtained. 

MODIFICATIONS  OF  THE  SLIDE  VALVE. 

The  ordinary  slide  valve  is  suitable  for  small  engines;  but  for 
large  sizes  some  method  must  be  employed  to  balance  the  steam 
pressure  on  the  back  of  the  valve.  With  large  valves,  such  for 
instance  as  those  of  locomotives  or  large  marine  engines,  a  great 
force  is  exerted  by  the  steam,  and  the  valve  is  forced  against  its 
seat  so  hard  that  a  large  amount  of  power  is  necessary  to  move  it. 
This  excessive  pressure  causes  the  valve  to  wear  badly  and  is  a 
dead  loss  to  the  engine.  The  larger  the  valve,  the  greater  this 
loss  will  be. 

Piston  Valve.  To  prevent  excessive  pressure  on  the  back  of 
the  valve,  the  piston  valve  is  commonly  used,  especially  in  marine 
engines.  This  valve  consists  of  two  pistons,  which  cover  and 
uncover- the  ports  in  precisely  the  same  manner  as  the  laps  of  the 
plain  slide  valve.  These  pistons  are  secured  to  the  valve  stem  in 
an  approved  manner  and  are  fitted  with  packing  rings. 

The  valve  seat  consists  of  two  short  cylinders  or  tiibes 
accurately  bored  to  fit  the  pistons  of  the  valve.  The  port  open- 
ings are  not  continuous  as  in  the  plain  slide  valve,  but  consist  of 
many  small  openings,  the  bars  of  metal  between  these  openings 
preventing  the  packing  rings  from  springing  out  into  the  ports. 

Steam  may  be  admitted  to  the  middle  of  the  steam  chest  and 
exhausted  from  the  ends  or  vice  versa.  With  the  former  method, 
the  live  steam  is  well  separated  from  the  exhaust,  and  the  valve- 
rod  stuffing  box  is  exposed  to  exhaust  steam  only.  This  is  a  good 
arrangement  for  the  high-pressure  cylinder;  if  used  for  a  cylinder 
in  which  there  is  a  vacuum,  air  may  leak  into  the  exhaust  space 
through  the  valve-rod  stuffing  box.  With  this  arrangement  the 
steam  laps  must  be  inside  and  the  exhaust  laps  on  the  outside  ends. 

The  piston  valve  may  be  laid  out  and  designed  by  means  of 
the  Zeuner  diagram  just  as  if  it  were  a  plain  slide  valve,  and  the 
action  is  the  same  except  that  it  is  balanced  so  far  as  the  steam 


VALVE  GEARS 


39 


pressure  is  concerned;  the  power  to  drive  it  being  only  that  neces- 
sary to  overcome  the  friction  due  to  the  spring  rings. 

Fig.  33  shows  a  section  of  the  piston  valve  and  the  high- 
pressure  cylinder  for  one  of  the  engines  of  the  IT.  S.  S.  "  Massa- 
chusetts." This  valve  consists  of  two  pistons  connected  by  a 
sleeve  through  which  the  valve  rod  passes.  This  valve  rod  is  pro- 

O  i  -I 

longed  to  a  small  balancing  piston,  placed  directly  over  the  main 


33. 


valve.  The  upper  end  of  the  balancing  cylinder  does  not  admit 
steam,  so  that  the  steam  pressure  below  the  balancing  piston  will 
practically  carry  the  weight  of  the  piston  valve,  thus  relieving  the 
valve  gear  and  making  the  balance  more  nearly  complete. 

DoubIe=Ported  Valve.  Sometimes  it  is'  impossible  to  get 
sufficient  port  opening  for  engines  of  large  diameter  and  short 
stroke,  especially  those  having  a  plain  slide  valve  with  short  travel. 


40 


VALVE  GEARS 


This  difficulty  may  be  overcome  by  means  of  the  double-ported 
valve  shown  in  Fig.  84.  It  is  equivalent  to  two  plain  slide  valves, 
each  having  its  laps.  The  inner  valve  is  similar  to  a  plain  slide 
valve  except  that  there  is  communication  between  the  exhaust 
space  and  the  exhaust  space  of  the  outer  valve.  Each  passage  to 
the  cylinder  has  two  ports;  a  bridge  separates  the  exhaust  of  the 
outer  valve  from  the  steam  space  of  the  inner  valve,  and  the  outer 
valve  is  made  long  enough 'to  admit  steam  to  the  inner  valve. 


Fig.  34. 

This  valve  may  be  considered  as  equivalent  to  two  equal  slide 
valves  of  the  same  travel,  each  having  one-half  the  total  port 
opening.  To  admit  the  same  amount  of  steam  as  a  plain  slide 
valve,  the  double-ported  valve  requires  but  half  the  valve  travel; 
this  is  advantageous  in  high-speed  engines. 


Fig.  35. 

To  balance  the  excessive  steam  pressure,  the  back  of  the  valve 
is  sometimes  provided  with  a  projecting  ring  which  is  fitted  to  a 
similar  ring  within  the  top  of  the  valve  chest.  These  rings  are 
planed  true,  and  fit  so  that  steam  is  prevented  from  acting  on  the 
back  of  the  valve.  The  space  inside  the  rings  is  sometimes  placed 
in  communication  with  the  condenser. 


VALVE  GEARS 


-II 


The  Trick  Valve.  The  defect  of  the  plain  slide  valve,  due  to 
the  slowness  in  opening  and  closing,  is  largely  remedied  in  the 
trick  valve,  which  is  so  made  that  a  double  volume  of  steam  enters 
during  admission.  Thus  a  quick  and  full  opening  of  the  port  is 
obtained  w4th  a  small  valve  travel. 

In  Fig.  35  the  valve  is  shown  in  mid-position.  It  is  similar 
to  a  plain  slide  valve  except  that  there  is  a  passage  PF  through  it. 
It  has  an  outside  lap  ()  and  an  inside  lap  I.  The  seat  is  raised 
and  has  steam  ports  SS,  bridges  BE,  and  exhaust  port  K.  If  the 
valve  moves  to  the  right  a  distance  equal  to  the  outside  lap  plus 
the  lead,  it  will  be  in  the  position  shown  in  Fig.  30.  Steam  will 
be  admitted  at  the  extreme  left  tdge  of  the  valve  just  the  same  as 
though  it  were  a  plain  slide  valve;  also,  since  steam  surrounds  the 
valve  it  will  be  admitted  through  the  passage  as  shown  in  Fig.  30. 


Fig-  36. 


Fig.  37. 


If  the  lead  is  the  same  as  for  a  plain  slide  valve,  T1^  inch  for 
instance,  this  valve  would  give  double  the  port  opening,  that  is  J 
inch,  wrhen  the  valve  was  open  a  distance  equal  to  the  lead. 

Fig.  37  shows  the  valve  when  it  is  in  its  extreme  position  to 
the  right  and  the  port  is  full  open  to  steam. 

Piston  valves  are  also  made  with  a  passage  similar  to  that  of 
the  trick  valve  for  double  admission.  The  valve  used  with  the 
Armington  and  Sims  engine  is  perhaps  the  best  example. 

Balanced  Valves.  Since  there  is  a  wide  difference  between 
the  pressure  of  admission  and  exhaust,  there  must  always  be  a 
great  pressure  acting  upon  the  valve,  causing  it  to  run  hard  and 
wear  excessively.  The  greater  the  steam  pressure,  the  lower  the 
pressure  at  exhaust  and  the  larger  the  valve,  the  greater  this  pres- 
sure will  be. 


42  VALVE  GEARS 


Piston  valves  are  commonly  used  on  the  higli  and  intermedi- 
ate cylinders  of  triple-expansion  engines,  and  if  well  made  and 
fitted  with  spring  rings,  should  not  leak.  Small  piston  valves  are 
often  made  without  packing  rings;  but  even  if  they  fit  accurately 
when  new,  they  soon  become  worn  and  cause  trouble. 

The  double-ported  valve,  the  trick  valve,  and  others  often  have 
some  device  for  relieving  the  pressure,  such  as  a  bronze  ring  or 
cylinder,  fastened  to  the  back  of  the  valve.  This  ring  is  pressed 
by  springs  against  a  finished  surface  of  the  valve  client  cover,  and 
the  space  thus  enclosed  by  the  ring  may  be  connected  to  the 
exhaust.  There  are  numerous  devices  for  balancing  valves,  but 

O 

they  are  usually  more  or  less  expensive  and  are  liable  to  cause 
trouble  from  leakage. 

STEPHENSON   LINK  MOTION. 

One  of  the  earliest,  and  at  present  one  of  the  most  common 
mechanisms  for  reversing  engines,  or  changing  the  ratio  of  expan- 
sion, is  the  Stephenson  link  motion,  shown  in  Fig.  38.  This  illus- 
tration is  taken  from  the  drawings  of  a  recent  battleship  engine, 
and  may  be  considered  the  typical  arrangement  of  the  Stephenson 
gear  as  applied  to  marine  practice. 

The  two  eccentrics  E  and  E',  whose  centers  are  at  C  and  C, 
respectively,  are  shown  in  their  relative  positions  when  the  crank 
OA  is  at  dead  center.  The  eccentric  rods  II  and  II'  are  connected 
by  forked  ends  to  the  link  pins  II  and  G.  The  link  consists  of 
two  curved  bars  bolted  together  in  such  a  manner  that  they  may 
slide  by  the  link  block  N.  On  the  link  are  three  sets  of  trunions; 
the  two  outer  ones,  or  link  pins,  are  fitted  into  the  forked  end  of 
the  eccentric  rods,  and  the  middle  one,  known  as  the  saddle  pin, 
is  fitted  into  the  end  of  the  drag  links  FM. 

The  valve  stem  has,  at  its  lower  end,  a  pivoted  block  N,  called 
the  link  block,  provided  with  slotted  sides  through  which  the  links 
can  slide  from  right  to  left.  The  reverse  shaft,  or  rock  shaft,  X, 
here  shown  in  full  gear  "forward,"  may  be  turned  until  F  moves 
over  to  B;  in  this  position  the  link  will  be  pushed  across  the  link 
block,  and  the  valve  will  get  its  motion  from  the  rod  II'  instead 
of  from  It  as  before.  The  link  in  this  position  would  be  full  gear 
4 'astern  ." 


VALVE  GEARS 


43 


Pig.  38 


44 


VALVE  GEARS 


In  all  large  engines,  such  as  marine,  the  reverse  shaft  is 
turned  by  power,  but  in  smaller  engines,  such  as  locomotives,  the 
engineer  can  turn  the  shaft  by  means  of  a  lever. 

When  set  full  gear  forward,  as  in  Fig.  38,  the  valve  admits 
steam  to  the  crank  end  of  the  cylinder,  and  the  crank  revolves  as 


Fig.  39. 

shown  by  the  arrow.  As  the  crank  turns,  both  eccentrics  impart 
motion  to  the  link,- but  the  "go  ahead"  link  pin  II  approximately 
coincides  with  the  link  block,  so  that  nearly  all  its  up-and-down 
motion  is  transmitted  to  the  valve  stem,  while  the  "  go  astern  " 


Pig.  40. 


eccentric  exerts  but  little  effect  upon  the  link  block.  Moving  the 
drag  links  over  to  the  extreme  right  reverses  all  these  conditions 
by  bringing  the  other  link  pin  under  the  link  block.  In  this  posi- 
tion, steam  will  be  admitted  to  the  other  end  of  the  cylinder,  and 
the  engine  will  run  in  the  opposite  direction.  This  will  be  clearly 
seen  by  referring  to  Fig,  38. 


VALVE  GEARS 


45 


When  at  full  gear,  either  forward  or  backing,  the  valve  moves 
as  if  there  were  really  but  one  eccentric,  while  at  intermediate 
points  its  motion  is  the  result  of  the  combined  influence  of  both 
eccentrics,  one  tending  in  a  measure  to  counteract  the  other.  The 
effect  of  this  is  to  shorten  the  valve  travel  the  same  as  if  the  valve 
were  driven  by  a  new  eccentric  having  less  throw  than  either  of 
the  other  two. 


Fig.  4L 


Decreasing  the  valve  travel  causes  cut-off  to  occur  earlier 
compression  is  earlier,  release  later,  and  the  lead  is  reduced  some- 
what. If  every  point  of  the  link  moved  in  the  arc  of  a  circle 
when  the  drag  link  is  shifted,  the  lead  would  not  alter;  but,  since 
the  eccentric  rods  about  which  each  end  swings  are  centered  at 
different  points,  C  and  C',  this  is  impossible. 

Figs.  39  and  40  show  the  two  principal  ways  of  arranging 
the  eccentric  rods  of  a  Stephenson  gear.  The  first  is  said  to  have 
"open  rods",  the  second  .i4  crossed  rods";  referring  to  whether 
the  rods  are  crossed  or  open  when  both  the  eccentrics  face  the  link. 
It  can  easily  be  seen  that  when  the  eccentrics  shown  in  Fig.  39 
have  turned  through  180°  they  will  be  in  the  position  shown  in 
Fig.  41,  but  this  is  the  same  arrangement  as  before  and  is  "  open  " 
rods.  The  full  lines  show  the  positions  in  full  gear  forward,  while 


VALVE  GEAKS 


the  dotted  lines  indicate  the  positions  in  mid  gear.  Witb  open 
rods  it  will  be  seen  that  when  at  full  gear  the  link  block  is  at  G, 
and  that  if,  \vithout  turning  the  crank,  the  link  is  shifted  to  mid 
gear,  then  the  link  block  moves  to  J,  Fig.  39,  and  the  valve  must 
consequently  be  moved  toward  the  right  an  amount  equal  to  GJ, 
thereby  increasing  the  lead  on  the  crank  end  of  the  cylinder. 
With  crossed  rods,  moving  the  link  from  full  to  mid  gear  moves 
the  link  block  from  G  to  J,  Fig.  40,  thus  reducing  the  lead.  It 
follows  then  that  open  rods  give  increasing  lead  from  full  toward 
mid  gear,  and  that  crossed  rods  give  decreasing  lead.  With  crossed 
rods  there  will  be  no  lead  when  in  mid  gear.  It  will  be  apparent 
that  the  shorter  the  rods  the  greater  this  increase  or  decrease 
will  be. 


REVERSE   ROD 


Fig.  42. 


Nearly  all  marine  engines,  and  some  English  locomotives, 
have  their  link  blocks  carried  directly  on  the  valve  rod.  Ameri- 
can locomotives  commonly  use  a  rocker,  one  end  of  which  carries 
the  link  block  while  the  other  moves  the  valve  rod.  This  arrange- 
ment indicated  in  Fig.  42  makes  it  possible  to  place  the  valve  and 
steam  chest  above  the  cylinder.  The  position  of  the  crank  for  the 
same  valve  position  is  just  opposite  that  shown  in  Fig.  39  because 
the  rocker  reverses  the  valve  motion;  this  gives  an  arrangement  of 
crank  and  eccentrics  that  is  identical  with  that  indicated  in  Fig. 
41  and  the  rods,  although  apparently  crossed,  are  in  reality  of  the 
open  rod  arrangement,  giving  increasing  lead  toward  mid  gear. 
A  rod  from  the  bell -crank  lever  on  the  reverse  shaft  E,  leads  back 
to  the  engineer's  cab  and  connects  with  the  reverse  lever.  This 
ever  moves  over  a  notched  arc,  and  may  be  held  by  a  latch  in  any 


VALVE  GEARS  47 


one  of  the  notches,  thus  setting  the  link  in  any  position  from  mid 
gear  to  full  gear,  either  forward  or  back. 

The  Stephenson  link  is  designed  to  give  equal  lead  at  both 
ends  of  the  cylinder;  but  to  accomplish  this,  the  radius  of  the  link 
arc  (that  is  an  imaginary  line  in  the  center  of  the  slot)  must  be 
equal  to  the  distance  from  the  center  of  this  slot  to  the  center  of 
the  eccentric.  In  Fig.  38  the  radius  of  the  link  arc  is  equal  to 
OH  and  C'G. 

Exact  quality  of  lead  is  not  essential,  and  the  radius  of  the 
link  arc  is  sometimes  made  greater  or  less  than  stated  above  in 
order  to  aid  in  equalizing  the  cut-off;  but  the  change  should  never 
be  great  enough  to  affect  the  leads. 

Stephenson  originally  intended  to  use  the  link  simply  as  a 
reversing  gear,  but  soon  found,  however^  that  at  intermediate 
points  between  the  two  positions  of  full  gear,  it  would  serve  very 
well  as  a  means  of  varying  the  expansion  and  cut-off.  Yery  soon 
the  link  came  to  be  used  not  only  on  locomotives  and  marine 
engines,  but  on  stationary  engines  as  well,  in  connection  wJth  the 
reverse  shaft  which  was  under  the  control  of  the  governor.  The 
mechanism  proved  to  be  too  heavy  to  be  easily  moved  by  a  gov- 
ernor and  it  has  gradually  fallen  into  disuse  on  stationary  engines 
excepting  as  a  means  of  reversing. 

In  marine  practice,  the  variable  expansion  feature  is  of  little 
value,  for  marine  engines  run  under  a  steady  load  and  the  link  is 
set  either  at  full  gear  or  at  some  fixed  cut-off.  For  locomotives, 
however,  the  variable  expansion  is  nearly  as  important  as  reversing. 
Locomotives  are  generally  started  at  full  gear,  admitting  steam  for 
nearly  the  entire  stroke,  and  then  exhausting  it  at  relatively  high 
pressure.  This  wasteful  use  of  stearn  is  necessary  to  furnish  the 
power  needed  in  starting  a  train.  After  the  train  is  under  way, 
less  power  is  required  per  stroke,  and  the  link  is  gradually  moved 
toward  mid  gear,  or  " notched  up"  by  the  engineer,  thus  hasten- 
ing the  cut-off;  the  expansion  is  increased  and  the  power  is  reduced 
in  proportion  to  the  load. 

As   the  cut-off  is   changed,   it  is    desirable  to   maintain   a 
approximately  equal  cut-off  at  each  end  of  the  cylinder;  this  can 
be  secured  in  the  Stephenson  gear  by  properly  locating  the  saddle 
pin  and  the  reverse  shaft,     When   used  without  a  rocker,  as  in 


48  VALVE   GEAKS 


Fig.  38,  the  saddle  pin  ehculd  be  on  the  arc  of  the  link  or  slightly 
ahead  of  it.  When  used  with  a  rocker,  the  saddle  pin  should  be 
behind  the  link  arcs,  and  to  give  symmetrical  action  for  forward 
and  backward  running,  it  should  be  opposite  the  middle  of  the  arc, 
that  is,  equally  distant  from  each  link  pin. 

The  Stephenson  link  cannot  be  designed  directly  from  the 
Zeuner  diagram,  but  a  systematic  investigation  can  be  made  by 
using  a  wocden  model  of  the  proposed  link.  This  can  be  mounted 
on  a  drawing  board,  and  the  effect  of  changing  the  position  of 
pins  and  the  proportions  of  rods  and  levers  can  be  determined 
without  difficulty.  By  a  system  of  trials  a  combination  can  be 
found  best  suited  to  obtain  the  desired  results.  Moreover,  a 
model  makes  it  possible  to  measure  directly  the  slip  cf  the  link 
block  along  the  link.  This  slip  should  be  kept  as  small  as  possi- 
ble to  prevent  rapid  wear.  It  can  be  controlled  to  some  extent  by 
properly  locating  the  link  pins,  by  avoiding  too  short  a  link,  and 
by  choosing  a  favorable  position  for  the  reverse  shaft. 

The  Gocch  Link.  Another  form  of  link  motion,  known  as 
the  Gooch  Link,  is  illustrated  in  Fig.  43.  It  has  been  extensively 
used  on  European  locomotives,  although  it  is  gradually  being 
replaced  by  a  type  of  valve  gear  known  as  the  Walschaert,  which 
will  be  described  later. 

The  Gooch  link  has  its  concave  side  turned  toward  the  valve 
instead  of  toward  the  eccentric.  The  radius  of  curvature  of  the 
link  is  equal  to  AB,  the  length  of  the  radius  rod.  The  link  is 
stationary  and  the  link  block  slides  in  the  link.  The  engine  is 
reversed  by  means  of  the  bell-crank  lever  on  the  reverse  shaft  E 
which  shifts  the  link  block  instead  of  the  link,  as  is  the  case  with 
the  Stephenson.  The  link  is  suspended  from  its  saddle  pin  JV1S 
which  is  connected  by  a  rod  to  the  fixed  center  F,  so  th^t  the  link 
can  move  forward  and  back  as  the  eccentricity  is  changed,  or  it 
can  pivot  about  its  saddle  pin  as  the  eccentrics  revolve. 

Since  the  radius  of  the  link  arc  is  equal  to  AB,  it  is  apparent 
that  the  block  can  be  moved  from  one  end  of  the  link  to  the  other, 
that  is,  from  full  gear  "  forward  "  to  full  gear  "  back  "  without 
moving  the  point  A,  which  is  on  the  end  of  the  valve  rod.  The 
lead  then  is  constant  for  all  positions  of  the  block,  and  the  distri- 
bution of  steam  for  locomotives  is  slightly  preferable  to  that 


VALVE  GEARS 


obtained  by  the  Stephenson  j  but  the  gear  is  more  complicated  and 
requires  nearly  double  the  distance  between  shaft  and  valve  stem. 

The  variable  lead  is  perhaps  a  slight  advantage  to  the  loco- 
motive, which  is  a  slow-speed  engine  in  starting,  thus  requiring 
but  little  lead.  As  the  speed  increases,  and  the  link  is  "  notched 
up  ",  the  lead  is  increased  as  the  cut-off  is  shortened,  and  at  high 
speed  we  have  a  large  lead.  With  the  Gooch  link,  the  lead  can  be 
set  for  the  average  running  speed,  and  although  a  little  too  great 
for  good  work  at  slow  speed,  it  is  a  matter  of  small  consequence, 
because  the  engine  runs  at  slow  speed  but  a  very  small  fraction  of 
the  time  it  is  in  service,  and  the  loss  due  to  large  lead  at  slow 
speed  is  of  no  consequence  whatever  in  a  day's  run. 

Several  other  link  motions  have  been  used;  but  at  the  present 
time  probably  more  Stephenson  link  motions  are  used  than  all 


Fig.  43. 

the  other  forms  of  reversing  gear  combined,  and  when  a  "  link 
motion  "  is  mentioned,  the  Stephenson  is  usually  meant  unless 
otherwise  specified. 

RADIAL  VALVE  GEARS. 

In  general,  it  would  be  desirable  to  have  precisely  similar 
steam  distribution  at  each  end  of  the  cylinder,  and  it  would  often 
be  of  great  advantage  with  an  expansion  gear  like  the  Stephenson, 
if  the  cut-off  could  be  shortened  without  changing  any  other  event 
of  the  stroke.  A  Stephenson  gear  can  be  made  to  maintain  equality 
of  lead  for  both  ends  of  the  cylinder  as  the  cut-off  is  shortened, 
but  we  have  seen  that  in  so  doing,  the  lead  of-  both  ends  is  either 


50 


VALVE   GEARS 


increased  or  diminished  according  as  the  link  is  arranged  with 
u  open  rods  "  or  "  crossed  rods  ".  Moreover,  the  compression  is 
hastened  by  bringing  the  link  to  mid-gear,  all  of  which  in  many 
instances  is  undesirable. 

This  disadvantage  of  the  Stephenson  link  motion  lead  to  the 
design  of  the  so-called  u  Radial  Valve  Gears  :',  many  of  which  are 
so  complicated  as  to  be  impracticable,  but  all  of  which  obtain  a 
fairly  uniform  distribution  of  steain. 


Fig.  44. 

Hackworth  Gear,,  The  essential  features  of  the  Hackworth 
Gear  are  indicated  in  outline  in  Fig.  44.  In  this  figure,  S  is  the 
center  of  the  shaft,  and  the  eccentric  E  is  set  180°  from  the  crank 
SH,r  At  the  right-hand  end  of  the  eccentric  rod  EA,  is  pivoted  a 
block  which  slides  in  a  straight,  slotted  guide.  The  guide  remains 
stationary  while  the  engiue  is  running,  but  can  be  turned  on  its' 


VALVE  GEARS 


51 


axis  P,  to  reverse  the  engine  or  change  the  cut-off.  P  is  a  pivot, 
located  on  the  horizontal  through  S  in  such  a  position  that 
DP  =  EA.  If  these  two  distances  are  equal,  A  will  coincide 
vvith  P  when  the  crank  is  at  either  dead  point  and  the  slotted 
guide  may  be  turned  from  "  full  gear  forward  ",  as  shown  in  the 
figure,  through  the  horizontal  position  to  "full  gear  backing",  as 


Fig  45 

shown  by  the  line  BL,  without  moving  the  valve.  Therefore  the 
leads  are  constant  for  all  positions  of  the  guide.  The  valve  rod 
running  upward  from  C,  connects  with  the  valve  stem  which  it 
moves  in  a  straight  line.  The  valve  stem  is  made  just  long 
enough  to  equalize  both  leads,  and  if  the  point  C  has  been  properly 
chosen,  the  two  cut-offs  will  be  very  nearly  equal  for  all  grades  of 
the  gear. 


52  VALVE  GEARS 


A  somewhat  better  valve  action  is  obtained  by  slightly  curv- 
ing the  slotted  guide,  with  its  convex  side  downward.  This  gear 
is  sometimes  used  on  marine  engines  and  on  small  stationary 
engines. 

Marshall  Gear.  The  most  objectionable  feature  of  the  Hack- 
worth  gear,  is  the  slotted  guide,  for  the  sliding  of  the  block  causes 
considerable  friction  and  wear.  The  Marshall  gear,  shown  in 
outline  in  Fig.  45,  is  designed  to  obviate  this  feature.  The  point 
A  moves  in  the  desired  path  by  swinging  on  the  rod  FA  about  F 
as  a  center.  While  the  engine  is  running,  the  lever  FP  remains 
stationary,  but  can  be  turned  on  its  axis  P  to  reverse  the  engine, 
or  change  the  cut-off.  The  pivot  P,  is  located  precisely  as  in  the 
Hackwwth  gear,  and  the  lever  FP  can  be  turned  from  u  full  gear 
forward",  as  showrn  in  the  figure,  to  "  full  gear  backing  ",  as  shown 
by  the  line  BP,  intermediate  positions  give  different  cut-offs  as 
with  the  Hackworth  gear.  Since'  FA  is  made  equal  to  FP,  the 
point  A  will  always  swing  through  P,  no  matter  where  F  may 
be,  and  will  coincide  with  P,  when  the  engine  is  on  dead  center. 
The  leads  therefore  will  remain  constant,  as  in  the  preceding  case. 

The  Marshall  gear  is  sometimes  made  with  C  at  the  right  of 
A  on  a  prolongation  of  the  line  EA.  In  this  case  if  the  same 
kind  of  valve  is  to  be  used,  the  eccentric,  E  must  move  with  the 
crank  instead  of  180°  from  it.  The  Marshall  gear  is  frequently 
used  on  marine  engines,  the  one  eccentric  being  simpler  than  the 
twro  required  by  the  Stephenson. 

Joy  Gear.  Perhaps  the  most  wridely  known,  and  certainly 
one  of  the  best  radial  gears  is  the  Joy,  outlined  in  Fig.  40.  It 
is  frequently  used  on  marine  engines  and  on  some  English  loco- 
motives. No  eccentrics  are  used,  the  valve  motion  being  taken 
from  C,  a  point  on  the  connecting  rod.  II  is  a  fixed  pivot  sup- 
ported on  the  cylinder  casting.  The  lever  ED  has  a  block  pivoted 
at  A,  which  slides  back  and  forth  in  a  curved  slotted  guide.  The 
guide  and  the  lever  PF  are  fastened  to  the  reverse  shaft  P,  and 
by  means  of  a  reverse  rod  leading  off  from  F,  can  be  turned  from 
full  gear  forward,  as  shown,  to  full  gear  backing  when  the  pin  F 
moves  over  to  B.  Motion  is  transmitted  to  the  valve  stem  by 
means  of  the  radius  rod  EG.  The  proportions  are  such  that  when 
the  crank  is  on  either  dead  point,  the  pivot  of  block  A  coincides 


VALVE  GEARS 


with  P,  so  that  the  curved  guide  may  then  be  set  in  any  position 
without  moving  the  valve;  therefore  the  leads  are  constant.  This 
gear  gives  a  rapid  motion  to  the  valve  when  opening  and  closing 
and  a  more  nearly  constant  compression  than  the  Stephen  son  gear, 
and  the  cut-off  can  be  made  very  nearly  equal  for  all  grades  of 
the  gear.  Its  many  joints  cause  wear  and  its  position  near  the 
crosshead,  makes  a  careful  inspection  of  the  crosshead  and  piston 
exceedingly- difficult  while  the  engine  is  running. 


Fig.  46. 

Walschaert  Gear.  This  radial  valve  gear,  although  seldom 
seen  in  the  United  States,  is  the  valve  mechanism  most  commonly 
used  on  locomotives  built  on  the  continent  of  Europe.  Like  all 
other  radial  gears,  it  gives  constant  lead,  and  a  distribution  of 
steam  very  nearly  alike  for  each  end  of  the  cylinder.  In  this 
respect  it  is  superior  to  the  Stephenson  link,  and  gives  without 
doubt  better  economy,  but  its  mechanical  construction  is  compli- 
cated, and  not  well  adapted  to  the  American  type  of  locomotive. 
Fig.  47  illustrates  this  type  of  gear.  S  is  the  center  of  the  driver 
axle.  The  crank  pin  K  has  forged  on  its  center  end  an  arm  KE, 
on  which  the  pin  E  is  fixed.  This  arm  lies  parallel  to  the  plane 
of  the  driving  wheels,  and  being  fixed  to  the  crank  pin,  turns  with 
the  wheel,  allowing  the  connecting  rod  to  pass  between  it  and  the. 
driving  wheels.  In  this  manner  the  point  E  moves  around  S  in  a 
circle,  and  moves  the  rod  EH  back  and  forth  just  as  if  it  were  an 
eccentric.  It  is  so  made  that  ES  is  perpendicular  to  the  crank 
KS,  and  therefore  the  action  of  the  pin  E  is  equivalent  to  an  eccen- 
tric with  no  angular  advance. 


VALVE  GEARS 


This  arm  reaching  back  from  the  outer  end  of  the  crank  pin 
is  one  of  the  most  objectionable  features  on  the  construction,  and 
is  sometimes  replaced  by  the  regular  type  of  eccentric  put  on  the 
shaft  between  the  driving  wheels. 

The  eccentric  rod  EH  causes  the  box  link  HP  to  oscillate  on 
fixed  trunions  P.  This  link  has  a  groove  curved  to  a  radius  equal 
to  GD,  the  length  of  the  radius  rod.  A  block  pivoted  at  G,  on 
one  end  of  the  radius  rod,  is  free  to  move  up  or  down  in  this 


TOP    OF  RAIL 


Fig  47. 

groove.  The  valve  derives  its  motion  from  C,  a  pivot  on  the  float- 
ing lever  CA.  Point  A  receives  motion  from  the  crosshead;  point 
D  from  the  eccentric  and  the  curved  link  ;  and  a  combination  of 
these  two  imparts  motion  to  C  (which  can  slide  only  along  the 
dotted  line).  A  bell-crank  lever  pivoted  above  the  link  shifts 
the  mechanism  from  •"  full  gear  forward  "  when  F  is  moved  to  B, 
thus  raising  G  above  the  link  pivot  or  saddle  pin. 

ADJUSTABLE  ECCENTRICS. 

The  position  of  an  eccentric  for  a  plain  slide  valve  is  90°  plus 
the  angular  advance  ahead  of  the  crank,  in  the  direction  in  which 
the  engine  is  to  turn.  Thus  A,  Fig.  48,  is  correctly  placed,  rela- 
tive to  the  crank  C,  if  the  engine  turns  right-handed.  For  run- 
ning in  the  opposite  direction,  the  position  of  the  eccentric  is  at 
D.  Some  engines  are  provided  with  a  reversing  mechanism  which 
causes  the  eccentric  to  shift  from  A  to  D,  either  along  the  arc 


VALVE  GEARS 


ABD,  or  along  the  straight  line  AED.  Sucli  engines  provide, 
not  only  for  reversing,  but  for  changing  the  cut-off  as  well  If 
the  eccentric  moves  on  the  arc  to  OB,  the  angular  advance  is 


Fig.  10. 

increased  and  all  the  events  of  the  stroke  are  hastened  as  well  as 
the  cut-off,  but  the  travel  of  the  valve  is  not  changed. 

Zeuner's  diagram,  Fig.  49,  is  lettered  to  correspond  with  Fig. 
48,  and  shows  the  effect  of  changing  the  angular  advance  from 
FOA  to  FOB.  If  OK  rep- 
resents the  lap,  the  crank 
angle  at  cut-off  will  decrease 
from  I1OK  to  IIOL,  and  the 
lead  will  increase  very  much, 
viz.,  from  GF  to  PIF.  If  the 
eccentric  is  shifted  on  the 
straight  line  to  E  (Fig.  48), 
a  different  valve  motion  will 
result.  The  angular  advance 
is  increased  as  before,  so  that 
all  the  events  are  hastened, 
but  the  eccentricity  is  now 
only  OE  instead  of  OB  and  Fio.  49 

the    valve    travel     is    conse- 
quently reduced.     Zeuner's  diagram  for  this  case,  Fig.  50,  shows  a 
decrease  in  crank  angle  at  cut-off  from  IOM  to  ION,    and  no 


nr, 


VALVE  GEARS 


change  in  the  lead  IF.     Let 

o 

us  consider  the  eccentric  posi- 
tion OA,  Fig.  48.  In  .this 
position  OI  represents  the 
displacement  of  the  valve 
from  mid-position  when  the 
engine  is  on  center.  If  the 
eccentric  moves  to  OB,  the 
displacement  will  be  OM, 
which  is  greater,  showing  an 
increase  in  lead  equal  to  IM, 
but  if  the  position  is  OE  in. 

stead  of  OB,  the  displace- 
Tig.  50. 

ment  from  mid-position  will 

be  OI  as  before.  It  is  evident  that  the  eccentric  can  move  on  the 
straight  line  from  A  to  I),  without  changing  the  lead,  while  the 
decreased  valve  travel  will  result  hi  an  earlier  cut-off.  If  the 


.  5L 


shifting  eccentric  is  to  be  used  for  an  automatic    cut-off,  as  in 
the  various  types  of  fly-wheel  governor,  the  curved  patli  is  not 


VALVE  GEAUS 


desirable  on  account  of  excessive  lead  at  short  cut-off,  but  if  it  is 
to  bo  used  only  as  a  means  of  reversing,  it  is  preferable  to  the 
straight  line. 

All  fly-wheel  governors  operate  by  shifting  the  eccentric, 
either  to  change  the  angular  advance,  the  travel  of  the  valve,  or 
both.  Fig.  51  illustrates  the  principle  of  a  governor  arranged  to 
give  decreasing  lead,  but  as  these  mechanisms  are  described  in 
the  Steam  Engine — Part  I,  under  the  head  of  governors,  a  further 
discussion  will  not  be  given  here. 

The  device  shown  in  Fig.  52  is  often  used  for  reversing 
engines  of  small  launches.  The  eccentric  E  is  loose  on  the  shaft 
between  a  fixed  collar  G,  and  a  hand  wheel  II.  A  stud  project- 
ing from  the  eccentric  passes  through  a  curved  slot  in  the  disc  of 
the  wheel,  and  can  be  clamped  by  a  hand  nut  F.  When  running 
forward  with  the  crank  at  C, 
the  center  of  the  eccentric  is  at 
A,  and  the  nut  clamped  at  F. 
To  reverse,  steam  is  shut  off, 
and  when  the  engine  stops,  the 
nut  F  is  loosened,  and  then 
moved  to  B  and  clamped;  or 
after  F  is  loosened,  the  wheel, 
shaft,  crank  and  propeller  are 
turned  over  by  hand  until  B 


strikes  the  stud  at  F,  where  it  is 
clamped.  The  engine  will  then 
run  astern. 

To  study  the  application  of 
the  Zeuner  diagram  to  this 
form  of  mechanism  turn  again 
to  Fig.  51.  If  OA  is  the  de- 
sired eccentricity  for  a  normal 
position  of  the  governor,  the  Fig  52> 

perpendicular  distance  of  A 

from  OF  is  made  equal  to  the  lap  OI,  plus  the  desired  lead.  Pivot 
D  is  then  located  equally  distant  from  A  and  I.  Zeuner's  dia- 
gram for  this  gear,  drawn  to  an  enlarged  scale,  is  shown  in  Fig.  58. 
The  angular  advance  FOB  is  laid  back  toward  ()(1.  OB  is  the 


58 


VALVE  GEARS 


maximum  eccentricity;  OI,  the  lap  or  the  desired  least  eccen- 
tricity. An  arc,  with  proper  radius,  described  through  B  and  I 
shows  the  path  of  the  eccentric.  If  the  eccentric  moves  in  to  A, 
the  crank  angle  at  cut-off  is  decreased  from  COD  to  COE,  and 
the  lead  decreased  from  FI  to  GI.  A  slight  decrease  in  lead  is 


Fig.  53. 

not  objectionable,  since  the  speed  is  not  allowed  to  increase  more 
than  two  or  three  per  cent;  and  further,  as  the  lead  increases, com- 
pression decreases,  so  that  one  influence  helps  to  counteract  the  other. 
The  decrease  in  maximum  port  opening  from  BII  to  AK  is  un- 
avoidable, but  it  is  permissible,  since  it  occurs  only  when  the  load 
decreases,  and  when  less  steam  should  be  admitted  to  the  cylinder. 

DOUBLE   VALVE  GEARS. 

It  has  been  shown  in  the  preceding  discussion,  that  a  plain 
slide  valve  under  the  control  of  a  gear  that  gives  a  variable 
cut-off,  such  as  a  shifting  eccentric  or  a  link  motion,  will  not 
give  a  satisfactory  distribution  of  steam  at  short  cut-off  owing 
to  execessive  compression,  variable  lead,  or  early  release.  These 
difficulties  are  overcome  in  a  measure  by  the  use  of  the  radial 
gear  ;  and  also  by  the  use  of  two  valves  instead  of  one.  The  main 


VALVE  GEARS 


59 


valve  controls  admission,  re- 
lease, and  compression  ;  the 
other,  called  the  cut-off  valve, 
regulates  the  cut-off  only, 
which  may  be  changed  with- 
out in  any  way  affecting  the 
other  events  of  the  stroke. 
This  cut-off  valve  may  be 
placed  in  a  separate  valve 
chest,  or  it  may  be  placed  on 
the  back  of  the  main  valve. 

Meyer  Valve.  The  most 
common  form  of  double  valve 
gear  is  the  Meyer  Yalve,  Fig. 
54.  The  cut-off  valve  is  made 
in  two  parts  and  works  on  the 
back  of  the  main  valve.  The 
two  parts  are  connected  to  a 
valve  spindle  with  a  right- 
and  left-hand  thread,  so  that 
their  positions  may  be  altered 
by  rotating  the  valve  spindle. 

A  swivel  joint  is  usually 
fitted  in  the  valve-spindle  be- 
tween the  steam  chest  and  the 
head  of  the  valve  rod,  and  the 
valve  spindle  prolonged  into 
a  tail  rod  which  projects 
through  a  stuffing  box  on  the 
head  of  the  steam  chest.  See 
Fig.  55.  The  end  of  this  tail 
rod  is  square  in  section  and 
reciprocates  through  a  small 
hand  wheel  by  means  of  which 
it  can  be  rotated  while  the  en- 
gine is  running,  whatever  the 
position  of  the  valve  may  be. 


60 


VALVE  GEARS 


Each  valve  is  under  the  control  of  a  separate  eccentric.  The 
eccentric  moving  the  main  valve  is  usually  fixed,  while  the  cut-off 
valve  eccentric  may  be  under  the  control  of  a  governor.  Since 
a  slight  compression  is  desired,  the  main  valve  is  set  to  give  late 
cut-off.  This  will  give  late  release  and  late  compression,  and  allow 
a  wide  range  of  cut-off  for  the  cut-off  valve.  With  this  gear,  lead, 
release,  and  compression  are  entirely  independent  of  the  ratio  of 
expansion,  and  the  cut-off  is  much  sharper,  because  the  cut-off 
valve,  when  closing  the  ports,  is  always  moving  in  a  direction 


Fig.  55. 

opposite  to  that  of  the  main  valve.     The  valve  may  be  designed 
by  means  of  Zeuner's  diagrams. 

Design  of  a  Meyer  Valve.  Let  us  design  a  Meyer  Yalve 
having  an  eccentricity  of  2  inches.  Let  the  eccentricity  of  the 
cut-off  valve  be  2J  inches  and  the  relative  travel  of  the  cut-off 
valve  in  relation  to  the  main  valve  be  3  inches.  This  will  make 
the  relative  motion  of  the  cut-off  valve  equivalent  to  the  .travel  of 
a  plain  slide  valve  with  an  eccentricity  of  1-J  inches.  Let  the  out- 
side lap  on  the  main  valve  be  |  inch,  the  lead  ^  inch,  the  com- 
pression 95  per  cent  of  the  stroke,  and  let  the  ratio  of  the  length 
of  the  crank  to  connecting  rod  be  six. 


VALVE  GEARS 


61 


In  Fig.  56  draw  XOY  equal  to  4  inches,  the  main  valve 
travel.  Lay  off  YD  =  95  per  cent  of  4  =  3.8  inches,  and  with 
a  radius  of  12  inches,  and  the  center  on  YX  produced  draw  the  arc 
DH.K.  H.K.O  is  the  crank  position  at  compression.  C.K.O,  the 
crank  position  at  cut-off,  is  found  in  a  similar  manner.  Lay  off  OI 
equal  to  the  lap  plus  the  lead,  and  draw  the  valve  circle  for  the 
main  valve  through  I  and  O  with  a  diameter  equal  to  its  eccen- 
tricity of  2  inches.  To  do  this  take  a  radius  equal  to  1  inch,  and 
draw  arcs  from  I  and  O  as  centers  that  shall  intersect  at  B.  B  is 


C.K. 


H, 

Fig.  56. 

tne  center  of  the  valve  circle  and  QBE  is  the  eccentricity,  2  inches. 
With  E  as  a  center,  and  with  a  radius  equal  to  half  the  relative 
travel  of  the  cut-off  valve  (in  this  case  1^  inches),  draw  an  arc. 
With  O  as  a  center  and  with  a  radius  equal  to  2^  inches,  the 
eccentricity  of  the  cut-off  valve,  draw  another  arc  intersecting  the 
first  one  at  F.  On  OF  as  a  diameter  construct  a  valve  circle. 
This  valve  Vill  represent  the  absolute  motion  of  the  cut-off  valve, 
independent  of  the  motion  of  the  main  valve.  This  circle  then 
will  show  the  displacements  of  the  cut-off  valve  from  the  center  of 
the  sterna  chest.  With  E  as  a  center  and  with  a  radius  equal  to 
FO  draw  an  arc,  and  with  O  as  a  center  and  with  a  radius  equal 


62 


VALVE  GEAKS 


to  EF  draw  another  arc  intersecting  the  first  at  G.  On  OG  as  a 
diameter  construct  a  valve  circle.  This  circle  will  then  represent 
a  travel  of  the  cut-off  valve  moving  on  the  main  valve.  That  is, 
,it  will  represent  the  displacements  of  the  cut-off  valve  from  the 
center  of  the  main  valve.  This  circle  is  not,  properly  speaking, 


MIN.CUT 
OFF  15 


H 


Fig.  57. 

a  valve  circle  and  OG  is  not  an  eccentricity,  but  simply  represents 
the  relative  motion  of  the  two  valves.  This  can  be  proved  by 
analytical  geometry,  but  an  inspection  of  the  figure  shows  that 
this  must  be  true. 

Draw  the  crank  line  OC  at  any  position,  cutting  the  valve 
circles  at  a  and  1>  and  c.  O^  represents  the  absolute  displace- 
ment of  the  cut-off,  valve,  that  is,  from  the  center  of  the  steam 
chest  and  Oc  represents  the  displacement  of  the  main  valve.  The 


VALVE  GEARS  68 


relative  displacement  of  the  cut-off  valve,  that  is,  from  the  center 
of  the  main  valve,  will  be  the  difference  between  Oc  and  O#,  since 
both  valves  are  moving  in  the  same  direction.  By  careful  meas- 
urement it  will  be  found  that  O£  =  Oc  -  O^,  and  any  ara  as  ()/> 
on  the  auxiliary  circle  OIG  will  correctly  represent  the  displace- 
ment of  the  cut-off  valve  from  the  center  of  the  main  valve  at  the 
corresponding  crank  angle. 

Fig.  57  shows  the  crank  angle  at  head -end  compression  H.K., 
and  at  crank-end  compression  O.K.,  the  main  valve  circle,  and  the 
auxiliary  circle  which  are  transferred  from  Fig.  56.  The  con- 
struction lines  and  all  lines  not  essential  to  the  figure  are  omitted 
to  avoid  confusion. 

Lay  off  on  Fig.  57,  OI  equal  to  the  outside  lap  J  inch  and 
draw  the  head-end  lap  circle  H.E.O.  It  will  intersect  the  valve 
circle  for  the  main  valve  at  L  and  M.  Through  L  draw  the  crank 
position  at  admission  (head-end)  H.A.  and  the  crank  position  at 
cut-off  through  M.  This  gives  the  greatest  possible  cut-off.  The 
cut-off  valve  may  be  set  to  give  a  much  earlier  cut-off  than  this, 
but  of  course  a  later  setting  would  be  of  no  avail  for  the  port 
would  be  closed  by  the  main  valve  at  this  angle.  The  crank  line 
OMH  cuts  the  auxiliary  circle  at  Jtf ,  so  that  ON  (1|J-  inches)  is 
the  clearance  of  the  cut-off  valve.  That  is,  the  edge  of  the  cut-off 
valve  must  be  set  1  if  inches  from  the  edge  of  the  main  valve  port 
in  order  to  cut-off  at  this  crank  angle.  The  full  lines  of  Fig.  54 
show  the  cut-off  valve  placed  in  this  position. 

The  intersection  of  H.K.O  with  the  lower  valve  circle,  gives 
the  inside  lap  at  the  head  end  of  the  cylinder.  This  line  comes  so 
nearly  tangent  to  the  valve  circle  that  the  intersection  can  be 
determined  only  by  dropping  a  perpendicular  to  H.K.O.  frqm  E1. 
This  cuts  the  circle  at  P  and  OF  —  ^  inch  equals  the  head-end 
inside  lap,  and  II.E.L  represents  the  corresponding  lap  circle. 

The  crank-end  angle  at  compression  is  O.K.  which  cuts  the 
upper  valve  circle  at  N',  giving  an  inside  lap  for  the  crank  end  of 
OK'=s=-JJ  inch.  To  make  this  intersection  more  apparent  the 
perpendicular  can  be  drawn  from  E  as  previously  explained. 

Suppose  that  it  was  required  that  the  minimum  cut-off  should 
be  15  per  cent.  Find  the  crank  position  at  15  per  cent  of  the  stroke 
in  the  same  manner  as  the  crank  position  was  found  at  compression, 


64  VALVE  GEARS 


Produce  this  line  through  O  until  it  cuts  the  auxiliary  circle  at  S. 
Then  OS  =  ||-  inch  =  the  required  lap  for  the  cut-off  valve  in 
order  to  cut-off  at  15  per  cent  of  the  stroke.  The  dotted  lines  in 
Fig.  54. show  the  cut-off  valve  drawn  in  this  position. 

For  a  valve  of  this  sort,  the  cylinder  port  should  be  1J  inches 
wide  and  the  valve  port  1  inch  wide.  Fig.  54  shows  this  valve  laid 
out  to  scale,  but  as  this  process  is  in  all  respects  similar  to  that 
described  for  laying  out  a  plain  slide  valve,  it  will  not  be  described 
in  detail. 

DROP  CUT-OFF  GEARS. 

\The  ordinary  slide  valve  controls  eight  different  events  of  the 
stroke,  that  is,  admission,  cut-off,  release,  and  compression  for  both 
ends  of  the  cylinder.  A  change  in  the  setting  of  a  plain  slide 
valve  that  affects  any  one  event  on  the  crank  end,  let  us  say,  will 
also  change  to  a  greater  or  less  degree  every  other  event  of  the 
stroke,  on  the  head  end  as  well  as  on  the  crank  end;  so  that  in 
setting  a  slide  valve  the  desired  position  for  one  event  must 
usually  be  sacrificed  in  order  to  make  the  others  less  objectionable. 
In  order  to  provide  a  better  distribution  of  steam  than  is  pos- 
sible with  a  single  valve,  some  engines  have  four  valves,  two  at 
each  end  of  the  cylinder.  In  horizontal  engines,  two  are  placed 
above  the  center  line  of  cylinder  and  two  below.  The  upper  are 
for  admission  and  cut-off,  the  lower  for  release  and  compres- 
sion. Since  each  valve  controls  but  two  events,  a  very  satisfactory 
adjustment  can  be  made  and  the  extra  complication  and  cost  for 
large  engines  are  more  than  overbalanced  by  the  advantages 
gained,  viz.:  A  very  much  better  distribution  of  steam,  short 
steam  passages  arid  small  clearances,  separate  ports  for  the  admis- 
sion of  hot  steam  and  the  exhaust  of  the  same  steam  after  expan- 
sion when  its  temperature  has  fallen,  and  finally  the  possibility  of 
opening  and  closing  the  ports  very  rapidly,  thus  preventing  wire- 
drawing. The  small  clearances,  short  ports  and  separate  admis- 
sion and  exhaust  materially  reduce  the  cylinder  condensation,  and 
thus  effect  a  large  saving  in  the  steam  consumption. 

When  four  valves  are  used  for  high  speeds,  the  motions  of 
all  must  be  positive,  that  is,  they  must  be  connected  directly  to 
some  mechanism  that  either  pushes  or  pulls  them  through  their 
entire  motion,  but  for  speeds  up  to  100  revolutions  or  so  a  disen- 


VALVE  GEARS 


65 


gaging  mechanism  may  be  used,  and  the  valves  may  shut  of  them- 
selves, either  by  virtue  of  their  weight  or  by  means  of  springs  or 
dashpots.  The  valve  is  usually  opened  by  means  of  links  or  rods, 
moved  by  an  eccentric,  and  at  the  proper  point  of  cut-off  the 
rod  is  disengaged  from  the  valve  which  drops  shut,  hence  the  term 
"  drop  cut-off  "  gears. 

Reynolds=Corliss  Gear.  The  most  widely  known  drop  cut- 
off gear  is  the  Eeyn olds- Corliss,  shown  in  Figs.  58  and  59;  it  is 
often  referred  to  as  the  Reynolds  hook-releasing  gear.  An  eccen- 
tric on  the  main  shaft  gives  an  oscillating  motion  to  a  circular  disc 


Fig.  68. 

called  the  wrist  plate,  pivoted  at  the  center  of  the  cylinder.  It 
transmits  motion  to  each  of  the  four  valves  through  adjustable 
links  known  as  steam  rods  or  exhaust  rods,  according  to  whether 
they  move  the  admission  or  exhaust  valves. 

The  valves  which  are  shown  in  section  in  Fig.  60  oscillate  on 
cylindrical  seats,  and  the  position  of  the  rods  is  so  determined  that 
they  give  a  rapid  motion  to  the  valve  when  opening  or  closing,  and 
hold  it  nearly  stationary  when  either  opened  or  closed. 

The  Reynolds  hook  is  shown  in  detail  in  Fig.  59.  The  steam 
arm  is  keyed  to  the  valve  spindle  which  passes  loosely  through  a 
bracket  on  which  the  bell-crank  lever  turns,  and  the  spindle  is 
packed  to  make  a  steam-tight  joint  where  it  enters  the  cylinder. 
Motion  of  the  steam  rod  toward  the  right  will  turn  the  fell-cran& 
lever  and  raise  the  hook  stud.  The  hook  (from  which  the  gear 
derives  its  name)  pivoted  on  thjs  stud,  has  at  one  end  a  hard- 


66 


VALVE  GEARS 


VALVE  GEARS  67 


ened  steel  die-  with  sharp,  square  edges,  and  at  the  other  end, 
a  small  steel  block  with  a  rounded  face.  As  the  hook  rises,  the 
hook  die  engages  the  stud  die  which  is  fastened  to  the  steam  arm, 
and  one  end  of  the  steam  arm  is  thus  raised.  This  turns  the 
valve  in  its  seat  and  admits  steam.  As  the  hook,  continues  to 
rise,  its  stud  moves  in  an  arc  above  the  valve  spindle,  and  the 
round-faced  block  at  its  left-hand  end  strikes  the  knock-off  cam 
which  causes  the  hook  to  turn  about  its  stud  and  disengage  the 
hook  die  from  the  stud  die.  In  raising  the  steam  Mrm,  the  dash- 
pot  rod  also  is  raised  and  a  partial  vacuum  is  created  in  the  dash- 
pot.  As  soon,  therefore,  as  the  dies  become  disengaged,  the 
dashpot  rod  quickly  drops  under  the  force  of  this  vacuum,  thus 
turning  the  steam  arm  and  closing  the  valve.  The  striking  of  the 
left-hand  end  of  the  hook  against  the  knock-off  cam  determines 
the  point  of  cut-off,  by  releasing  the  valve  at  that  instant. 

This  cam  is  a  part  of  the  knock-off  lever  to  which  the  governor 
cam  rod  is  fastened.  Any  action  of  the  governor  which  would 
cause  the  cam  rod  to  move  toward  the  right  would  cause  this 
knock-off  lever  to  turn  on  its  axis,  the  steam  arm,  and  conse- 
quently lower  the  position  of  the  knock-off  cam.  This  would 
cause  an  earlier  contact  between  the  cam  arid  end  of  hook,  and 
consequently  an  earlier  cut-off.  By  lengthening  or  shortening 
the  governor  cam  rod,  the  point  of  cut-off  can  be  adjusted  to  suit 
the  engine  load  without  changing  the  speed. 

There  is  a  limit  to  this  adjustment,  for  it  can  be  shown  that 
a  Corliss  gear  operated  by  a  single  eccentric  cannot  be  arranged  to 
cut-off  later  than  half  stroke.  Suppose  the  eccentric  is  set  just  90° 
ahead  of  the  crank.  Then  it  will  reach  its  extreme  position  just 
as  the  piston  gets  to  half  stroke.  If  by  that  time  the  hook  which 
was  rising  and  opening  the  admission  valve,  has  not  yet  struck 
the  knock-off  cam,  it  cannot  strike  it  at  all,  for  any  further  motion 
will  cause  the  hook  to  descend  to  its  original  position,  that  is  its 
position  at  the  beginning  of  the  stroke;  the  hook  will  not  disen- 
gage from  the  steam  arm  stud  at  all  and  the  bell  crank  will 
return,  closing  the  valve  in  the  same  manner  in  which  it  opened 
it.  Cut-off  will  then  take  place  near  the  end  of  the  stroke,  but  it 
will  not  be  the  sharp  cut-off  produced  by  the  sudden  drop  when 
the  dies  are  disengaged. 


VALVE  GEARS 


If  the  eccentric  were  set  less  than  90°  ahead  of  the  crank,  the 
cut-off  could  be  arranged  to  occur  later  than  half  stroke,  but  this 
is  decidedly  impracticable,  for  with  such  a  position  of  the  eccen- 
tric the  action  of  the  valves  at  release  and  compression  is  spoiled. 
When  it  is  necessary  to  cut-off  later  than  half-stroke,  as  some- 
times happens  on  low-pressure  cylinders  of  compound  engines,  it 
may  be  arranged  for  by  means  of  two  eccentrics,  one  set  more 
than  90°  ahead  of  the  crank  to  operate  the  exhaust  valves,  and  one 
less  than  90°  ahead  to  operate  the  admission  valves.' 2sj 

The  safety  cam  shown  in  Fig. 
59  is  an  important  part  of  a  Cor- 
liss gear.  If  for  any  reason  the 
engine  governor  should  fail  to 
act,  due,  for  instance,  to  the 
breaking  of  its  driving  belt,  the 
governor  would  drop  to  its  low- 
est position,  supply  more  steam 
to  the  engine  and  allow  it  to 
run  away.  The  safety  cam  pre- 
vents this  by  moving  so  far  to 
the  right  that  it  strikes  the  hook 
when  it  descends  to  pick  up  the 
steam  arm.  The  hook  is  conse- 
quently turned  toward  the  right 
and  then  lifted  without  engaging 
the  stud  die;  the  valve  conse- 
quently remains  closed  and  the  engine  stops. 

Brown  Releasing  Gear.  In  addition  to  the  Reynolds  hook, 
several  other  devices  are  in  use  for  opening  and  releasing  Corliss 
admission  valves.  Among  them  the  Brown  releasing  gear  shown 
in  Fig.  01  may  be  noted.  The  steam  rod  and  dashpot  rod  are 
arranged  much  the  same  as  in  the  Reynolds  gear.  The  governor 
cam  rod  operates  a  plate  cam  having  a  curved  slot  so  shaped  that  it 
takes  the  place  of  both  the  knock-off  and  the  safety  cam  of  Fig.  59. 
The  steam  arm  is  keyed  to  the  valve  spindle  and  carries  at  its 
lower  end  a  steel  die  which  is  free  to  slip  up  and  down  a  small 
amount.  The  part  of  this  gear  corresponding  to  the  Reynolds 
bell  crank  becomes  a  straight  rocker  pivoted  at  its  middle; 


Fig.  60. 


VALVE  GEARS 


and  the  part  corresponding  to  th.e  Reynolds  Look  has  at  one  end 
a  die  which  engages  the  die  of  the  steam  arm,  and  at  its  other 
end  a  roller  running  in  the  curved  cam  slot.  This  hook  is  really 
a  bell-crank  lever  with  arms  that  are  not  in  the  same  plane.  The 
bearing  on  which  it  turns  is  carried  on  the  lower  end  of  the 
rocker,  and  therefore  is  equivalent  to  a  movable  pivot  similar  to 
the  hook  stud  of  the  Reynolds  gear. 


VALVE 
SP1NDL 


CAN*         VSTEAM   ROD 


GOVERNOR    CAM   ROD 


Fig.  61. 

In  the  position  shown  the  dies  are  engaged.  Motion  of  the 
steam  rod  toward  the  right  will  move  the  lower  end  of  the  rocker 
toward  the  left,  and  consequently  turn  the  valve  spindle  in  a  right- 
handed  direction.  This  will  open  the  valve  and  at  the  same  time 
raise  the  dashpot  rod.  Mean  while,  the  roller  is  moving  toward 
the  left  in  a  circular  part  of  the  cam  slot,  the  center  of  which  is 
at  the  center  of  the  valve  spindle.  This  causes  the  steam  arm  and 
the  bell-crank  lever,  which  has  the  roller  at  one  end,  to  move  in 
such  a  way  that  there  is  no  relative  motion  between  them.  As 
soon,  however,  as  the  roller  comes  to  the  point  where  it  is  forced 
to  move  out  of  this  circular  path  and  move  farther  from  the  valve 
spindle,  both  arms  of  the  bell-crank  lever  are  turned  downward, 


70 


VALVE   GEARS 


the  dies  become  disengaged,  and  the  dashpot  closes  the  valve. 
The  slight  up-and-down  motion  of  the  steam-arm  die  allows  it  to 
rise  while  the  hook  die  passes  underneath  when  returning  to  re- 
engage for  the  next  stroke.  The  makers  claim  that  this  gear  per- 
mits a  much  higher  speed  than  is  possible  with  other  Corliss  gears. 
Greene  Gear.  Another  well-known  drop  cut-off  gear  is  the 
Greene,  shown  in  Fig.  62.  The  valves  are  of  the  gridiron  type, 
sliding  on  horizontal  seats,  the  admission  valves  parallel  to,  and 
the  exhaust  valves  at  right  angles  to  the  axis  of  the  cylinder  and 
just  below  it.  AA  are  rock  shafts  turning  in  fixed  bearings. 


Fig.  62. 

BE  are  the  admission  valve  stems.  C  is  a  slide  bar,  receiving  a 
reciprocating  motion  from  an  eccentric.  TT  are  tappets  connected 
to  the  slide  bar.  They  move  to  and  fro  with  the  slide  bar  and  can 
also  move  independently  up  and  down.  They  are  made  fast  at 
their  lower  end  to  the  gauge  plate  D  which  slides  through  the 
guide  E.  The  guide  E  is  made  fast  to  the  governor  rod  F  and 
through  this  means  can  be  raised  or  lowered,  thus  regulating  the 
height  of  the  tappets. 

As  the  slide  bar  moves  toward  the  right,  the  right-hand 
tappet  is  brought  into  contact  with  the  toe  of  the  rocker,  causing 
it  to  turn  on  its  bearings  and  move  the  rock  lever  and  the  valve 
stem  B  toward  the  right,  thus  opening  the  admission  valve.  Since 


VALVE  GEARS 


71 


the  tappet  moves  in  a  horizontal  direction  while  the  toe  of  the 
rocker  moves  in  an  arc,  it  will  be  seen  at  once  that  they  will  soon 
become  disengaged  and  release  the  valve  which  is  at  once  closed  by 
a  dash  pot  not  shown  in  the  figure.  If  the  governor  raises  the 
tappets,  cut-off  will  be  later.  A  nut  at  the  bottom  of  the  governor 
rod  allows  a  proper  adjustment  of  the  guide  and  gauge  plate.  As 
the  slide  plate  C  moves  toward  the  right,  the  left-hand  tappet  comes 
in  contact  with  the  heel  of  the  left-hand  rocker,  both  being  beveled, 
it  rises  in  its  socket  allowing  the  tappet  to  pass  under.  It  then 
falls  by  its  own  wreight  and  is  ready  to  engage  the  tappet  on  its 


Fig.  63. 

return  and  open  the  valve.  In  this  gear  the  disengagement  of  the 
valve  throws  no  load  whatever  on  the  governor  which  is  a  distinct 
advantage  over  the  Corliss  gear.  The  action  of  the  exhaust  valves 
is  not  shown  in  the  cut. 

The  Sulzer  Gear  is  a  drop  cut-off  widely  used  in  Europe. 
The  valves  are  of  the  poppet  type,  lifting  straight  from  conical 
seats,  so  that  there  is  no  friction.  They  are  usually  placed  verti- 
cally above  and  below  the  cylinder  axis  and  are  operated  by  eccen- 
trics from  a  shaft  geared  to  the  main  shaft.  The  admission  valves 
are  lifted  from  their  seats  by  suitable  levers,  then  released  by  a 


72 


VALVE  GEARS 


device  equivalent  in  action  to  the  Keynolds  hook  and  are  quickly 
closed  by  the  action  of  springs. 

The  exhaust  valves  of  all  drop  cut-off  gears  are  comparatively 
simple  in  their  operation  and  both  in  opening  and  closing  they  are 
moved  by  the  direct  action  of  the  exhaust  rods. 

A  common  form  of  vacuum  dashpot  for  closing  admission 
valves  is  shown  in  Fig.  G3.  The  rod  coming  down  from  the  steam 
arm  makes  a  ball-and-socket  joint  with  the  dashpot  piston.  The 
dashpot  is  often  let  down  into  the  engine  frame  as  shown.  When 
lifted,  the  piston  produces  a  partial  vacuum  underneath  it  so  that 
it  tends  to  drop  quickly  as  soon  as  the  valve  gear  is  released.  On 
some  of  the  largest  modern  engines  where  the  valves  are  very 
heavy,  steam-loaded  dashpots  are  used;  that  is,  the  dashpot  piston 
has  steam  pressure  on  one  side,  and  an  air  cushion  on  the  other 
prevents  it  from  striking  the  bottom  of  the  dashpot. 

Corliss  Valve  Setting.     The  setting  of  a  Corliss  valve  gear 

is  a  much  longer  process  than 
the  setting  of  a  plain  slide  valve, 
but  is  nevertheless  a  compar- 
atively simple  matter,  for  the 
various  adjustments  are  nearly 
all  independent  of  one  another. 
In  gears  like  that  shown  in  Fig. 
58  the  length  of  both  the  eccen- 
tric rod  and  carrier  rod  are' 
unusually  adjustable,  and  the 
former  should  be  of  such  length 
that  the  carrier  arm  swings  equai  distances  on  each  side  of  a  verti- 
cal line  through  its  pivot,  and  the  carrier  rod  should  be  adjusted 
until  the  wrist  plate  oscillates  symmetrically  about  a  vertical  line 
through  its  pivot  Nearly  all  Corliss  engines  have  one  mark  on 
the  wrist  plate  hub  and  three  on  the  wrist  plate  stand,  as  shown 
in  Fig.  64,  and  the  wrist  plate  should  swing  so  that  A,  the  mark 
it  carries,  moves  from  C  to  D,  but  not  beyond  either  one.  When 
A  is  in  line  with  B,  the  wrist  plate  is  in  mid-position.  The  valves 
are  then  not  in  their  exact  mid-position,  but  it  is  customary  to  regard 
them  as  being  in  mid-position,  and  to  speak  of  the  lap  as  the  amount 
the  valve  covers  the  port  when  the  wrist  plate  is  in  mid-position. 


WRIST  PLATE 
STAND 


Fig    64. 


VALVE  GEARS 


To  set  the  valves,  remove  the  bonnets  or  covers  of  the  valve 
chambers  on  the  side  opposite  the  gear.  The  ends  of  the  valves 
are  circular,  but  inside  their  cross-section  is  as  shown  in  Fig.  65. 
On  the  end,  in  line  with  the  finished  edge  of  the  valve,  and  on 
the  seat  in  line  with  the  edge  of  the  steam  port,  are  marks  as 
shown  in  Fig.  65.  When  these  marks  coincide,  the  valve  is  either 
just  opening  or  just  closing,  and  when  in  any  other  position,  the 
amount  of  opening  or  the  amount  by  which  the  port  is  closed  is 
shown  directly  by  the  distance  between  the  marks.  Block  the 
wrist  plate  in  mid-position,  hook  up  the  admission  valves  and 
adjust  the  length  of  the  steam  rods  by  means  of  the  right  and  left 
threads  provided  for  the  purpose,  until  the  ports  are  covered  by 
the  amount  of  lap  indicated  in  the  following  table  opposite  the 
given  size  of  engine. 


Dia.  of  Cyl. 
in  inches. 

12 
14  to  16 

18  to  22 
24  to  28 
30  to  36 
36  to  42 


Steam  Lap 
in  inches. 


Exhaust  Clearance  - 
in  inches. 


i 
A 


7V 
TrV 

TV 

A 


Next  adjust  .the  exhaust  rods  until  the  exhaust  ports  are  open 
an  amount  equal  to  the  clear- 
ance given  in  the  above  table. 
Set  the  engine  on  its  head- 
end  dead  point,  hook  the  car- 
rier  rod  onto  the  wrist  plate 
and  in  the  direction  in  which 
the  engine  is  to  run,  turn  the 
eccentric  enough  to  open  the   MARKS 
head-end  admission  valve  by 
a   proper   amount    of    lead ; 
then  the  eccentric  will  be  90° 
plus     the     angular    advance 
ahead    of    the    crank.      The 
proper  amount  of  lead  will  depend  upon  both  the  design  of  the  gear 
and  the  speed  at  which  the  engine  is  to  run  ;  and  may  vary  from 
¥y  for  small  engines  to  as  much  as  -£-%"  or  ^"  for  large  and  higher- 


Fig.  65. 


74  VALVE  GEARS 


speed  engines.  When  the  proper  amount  of  lead  has  been  obtained, 
fasten  the  eccentric  on  the  shaft  by  means  of  the  set  screw  and 
make  sure  by  trial  that  the  wrist  plate  moves  to  its  extremes  of 
travel.  The  dash  pot  rods  must  be  adjusted  so  that  when  the  dash- 
pot  piston  is  at  its  lowest  position,  the  hooks  (see  Fig.  59)  descend 
just  far  enough  for  the  hook  dies  to  snap  over  the  stud  dies  with 
about  -fa"  to  rV"  to  spare?  depending  on  the  size  of  the  gear. 

To  adjust  and  equalize  the  cut-off,  lift  the  governor  to  about 
the  position  that  it  will  occupy  when  running  at  normal  speed, 
and  put  a  block  under  the  collar  to  hold  it  in  this  position.  First 
set  the  double  lever  at  the  right  of  the  governor  cam  rods  so  that 
it  makes  approximately  equal  angles  with  each  rod,  and  then  turn 
the  engine  over  by  hand  until  the  piston  has  moved  to  the  desired 
point  of  cut-off.  Adjust  the  proper  cam  rod  until  the  knock-off 
cam  strikes  the  hook  and  allows  the  valve  to  close,  then  turn 
the  engine  to  the  point  of  cut-off  on  the  other  stroke  and  adjust  the 
other  cam  rod  in  a  similar  manner.  Now  set  the  governor  in  the 
lowest  position  to  which  it  could  fall  if  there  were  no  load  on 
the  engine,  and  set  the  safety  cams  so  that  in  this  position  the  hook 
cannot  open  the  valve.  A  latch  is  provided  on  which  the  governor 
can  be  supported  slightly  above  its  lowest  position  so  that  the  valves 
can  be  opened  by  the  hook  when  starting  the  engine.  As  soon  as 
the  engine  speeds  up  this  latch  must  be  moved  aside,  so  that  if  the 
governor  fails  to  act,  it  can  drop  to  its  lowest  point,  otherwise  this 
latch  would  hold  it  just  high  enough  so  that  the  safety  cams 
could  not  act. 

When  Corliss  gears  are  set  as  here  described,  the  position  of 
the  eccentric  may  not  be  quite  right,  due  to  an  incorrect  estimate 
of  the  amount  of  lead  required.  The  error  is  likely  to  produce 
faulty  release  and  compression  as  well  as  poor  admission,  but  it 
cannot  be  very  serious,  and  the  engine  will  turn  over  with  its  own 
steam,  so  that  indicator  diagrams  may  be  taken.  The  final  adjust- 
ments  can  then  be  determined  from  an  examination  of  the  diagrams. 


STEAM  ENGINE  INDICATORS 


A  most  important  question  concerning  a  steam  engine  is, 
"What  is  its  horse-power?  "  or  u  How  much  work  will  it  do  in  a 
given  time  ?  " 

.    Work  is  defined  as  pressure,  force,  or  resistance  multiplied  by 
the  distance  through  which  it  acts. 

Power  is  work  done  in  a  specified  time. 

In  the  steam  engine,  steam  is  the  agent  by  means  of  which 
heat  is  transformed  into  mechanical  work.  It  is  the  heat  in  the 
steam  that  does  the  work,  not  the  steam  itself. 

Work  is  obtained  from  the  heat  in  steam  by  confining  it  in  a 
closed  cylinder  which  is  fitted  with  a  piston  and  a  piston-rod. 
Steam  is  admitted  at  one  side  of  the  piston  while  the  other  is  open 
to  the  atmosphere  or  in  communication  with  a  condenser.  The 
pressure  of  steam,  usually  75  to  150  pounds  per  square  inch,  forces 
the  piston  to  the  other  end  of  the  cylinder,  driving  out  the  low- 
pressure  steam  in  front  of  it.  When  it  arrives  at  the  other  end, 
steam  is  admitted  to  that  end  and  the  piston  is  driven  back. 

The  piston  moves  because  the  pressure  on  one  side  is  greater 
than  that  on  the  other. 

In  order  to  move  the  piston,  work  must  be  performed.  The 
amount  of  work  is  easily  found,  since  work  equals  total  pressure 
multiplied  by  the  distance  through  which  the  piston  moves. 

Suppose  a  piston  is  2  square  feet  in  area  and  steam  at  a 
pressure  of  64.7  pounds  per  square  inch  acts  on  it  during  the 
entire  stroke  of  4  feet ;  the  other  side  of  the  piston  being  in  com- 
munication with  the  atmosphere.  The  total  pressure  is  then 
2  X  144  X  64.7  =  18,633.6  pounds.  If  this  pressure  acts 
through  4  feet  it  is  evident  from  the  definition  that  the  work  done 
per  stroke  will  be, 

18,633.6  X  4  =  74,534.4  foot-pounds. 


INDICATORS. 


Another  method  is  as  follows:  The  pressure  on  the  above 
piston  is  64.7  X  144  =  9316.8  pounds  per  square  foot.  The 
volume  swept  by  the  piston  during  one  stroke  is  2  X  4  z=  8  cubic 
feet.  If  we  multiply  the  pressure  per  square  foot  by  the  volume 
or  9316,8  by  8,  we  get  74534.4  foot  pounds,  the  same  result  as, 
before.  Thus  we  see  that  work  equals  unit  pressure  multiplied 
by  volume. 

Let  P  --  pressure  ou  the  piston  in  pounds  per  square  foot. 
p  =:  pressure  on  the  piston  in  pounds  j^er  square  inch. 
A  =  area  of  piston  in  square  inches. 
L  =  length  of  stroke  in  feet. 

V  —  volume  swept  by  piston  in  one  stroke  in  cubic  feet. 
W  =  work  done  in  foot-pounds. 
Then  from  the  above  example, 

Work  =  unit  pressure  multiplied  by  volume, 
or,     W  =  P  X  V 

It  is  evident  that  P  =  144  »,  and  V  =  -J^_,  X  L. 

144 

Then  we  have  these  expressions  for  work, 

W  =  P  X  V  =  144  p  X  V  =  144  p  X    A   x  L  =  p  L  A. 

Suppose  steam  is  admitted  to  the  cylinder  during  the  whole 
stroke,  as  in  the  above  example,  that  is,  one  end  of  the  cylinder  is 
in  communication  with  the  boiler.  The  other  end  is  open  to  the 
atmosphere.  If  we  draw  two  lines  at  right  angles  to  each  other, 
as  O  Y  and  O  X  in  Fig.  1,  the  volume  of  steam  for  any  position 
may  be  represented  by  some  distance  measured  on  the  line  O  X. 
Similarly  the  pressure  of  the  steam  at  any  position  of  the  piston 
may  be  represented  by  the  length  of  a  vertical  line  parallel  to  the 
line  O  Y. 

In  the  above  example,  the  area  of  the  piston  was  2  square 
feet,  the  length  of  stroke  4  feet  and  the  pressure  by  gage  50 
pounds.  Then  we  let  O  A  =.  the  atmospheric  pressure  =14.7 
pounds.  At  the  beginning  of  the  stroke  the  pressure  (absolute) 
is  14.7  _|_  50  =  64.7  pounds,  represented  by  the  distance  O  B,  or 
A  B  =-  50  pounds  pressure.  When  the  piston  has  passed  through 
i  of  the  stroke  it  is  represented  as  the  point  1,  or  B  1  is  the 
volume  swept  through  when  the  piston  has  completed  ^  of  the 


OTDICATOBS. 


stroke.  At  this  point  the  pressure  is  also  64.7  pounds  as  repre- 
sented at  1'.  Similarly,  when  the  piston  is  at  2,  3,  and  4  the  cor- 
responding pressure  is  2',  3',  4'.  Since  the  pressure  is  constant 
the  line  B  D  is  parallel  to  O  X.  We  see  from  the  above  that  50 
pounds  is  the  net  pressure  acting  on  the  piston  during  the  stroke, 
and  is  represented  by  A  B  and  lines  parallel  to  it.  The  volumes 
are  represented  by  the  horizontal  lino  A  C.  Then  since  W  =  P 
X  V  it  also  equals  O  B  X  O  X  which  is  evidently  the  area  of  the 
rectangle  O  B  D  X.  The  area  of  the  rectangle  O  B  D  X  is  pro- 
portional to  the  work  done  by  the  steam. 

In  Fig.  1,  one  inch  on  the  line  O  Y  =  40  pounds,  then  O  B 
is  1.6175  inches  long 
since  it  represents  64.7 
pounds.  Similarly  O  A 
must  be  .3675  inch  since 
it  represents  14.7 
pounds.  The  line  A  C 
is  2  inches  long;  then 
referring  to  the  preced- 
ing example,  one  inch 
in  length  =  |  =  4  cubic 
feet. 

Since  the  rectangle 
O  B  D  X  is  1.6175  by 


Fig.  1. 


2  inches,  the  area  is  3.235  square  inches.      But  one  inch  in  height 

equals  40  pounds  pressure  and  one  inch  in  length  equals  2  cubic 

feet.     Then  p  V  —  40  X  3.235  X  4  =  517.6  foot-pounds  and, 

W  =  144  p  V  --=  517.6   X   144  =  74,534.4  foot-pounds. 

In  the  above  cylinder  the  pressure  acting  on  one  side  of  the 
piston  was  64.7  pounds  per  square  inch.  There  was  also  a  press- 
ure of  14.7  pounds  per  square  inch  (the  atmospheric  pressure)  act- 
ing in  the  opposite  direction.  Then  the  work  done  against  the 
steam  pressure  is  represented  by  the  area  O  A  C  X  arid  is  equal 
to  144  p  V  =  144  X  14.7  X  8  =  16934.4  foot-pounds.  Then 
since  O  B  D  X  represents  the  total  work  done  on  one  side  of  the 
piston  and  O  A  C  X  represents  the  work  done  against  the  piston 
the  difference  A  B  D  C  represents  the  net  work.  This  net  work 
is  represented  by  the  shaded  area.  Also  if  the  amount  of  work  done 


INDICATORS. 


on  the  piston  is  74,534.4  foot-pounds  and  the  work  done  against 
the  piston  is  16,934.4  foot-pounds,  the  net  work  is  the  difference, 
or  57,600  foot-pounds. 

In  this  theoretical  discussion  the  same  result  may  be  obtained 
by  subtracting  the  atmospheric  pressure  or  back  pressure  from  the 
absolute  initial  pressure  and  using  the  difference  as  the  value  of 
p.  This  value  of  p  is  called  the  mean  effective  pressure. 

Then  64.7  --  14.7  =  50  and 

W  =  144  p  V  =  144  X  50  X  8  =  57,600  foot-pounds. 

The  area  is  proportional  to  the  work  done  whatever  the 
shape  may  be ;  provided  the  line  B  D  represents  the  relation 
between  pressures  and  volumes  on  the  steam  side  of  the  piston 
and  the  lower  line  A  C  represents  the  relation  between  pressures 
and  volumes  on  the  exhaust  side.  If  the  engine  is  of  the  con- 
densing type  the  line  A  C  will  be  nearer  O  X,  which  is  the  line 
representing  absolute  vacuum. 

Whatever  the  shape  of  the  diagram,  the  area  is  equal  to  the 
area  of  a  rectangle  of  the  same  length  and  a  height  equal  to  the 
mean  height,  or  mean  ordinate  as  it  is  called.  The  mean  ordinate 
represents  the  mean  or  average  net  pressure  on  the  steam  side  of 
the  piston.  Then  we  can  follow  these  rules  in  finding  the  work 
of  the  steam  from  the  diagram. 

Multiply  the  area  in  square  inches  by  the  scale  of  pressures, 
by  the  scale  of  volumes  and  by  144,  or  ; 

Multiply  the  length  of  the  mean  ordinate  by  the  scale  of  press- 
ures a  by  the  length  of  stroke,  and  this  product  by  the  area  of  the 
piston  in  square  inches. 

Example :  The  area  of  a  diagram  A  B  D  C  like  that  of  Fig. 
1  is  6.3  square  inches  and  its  length  is  3  inches.  The  scale  of 
pressure  is  30  pounds  per  inch  and  the  scale  of  volumes  is  1.99985 
cubic  feet  to  the  inch.  If  the  piston  is  20  inches  in  diameter  and 
the  length  of  stroke  2-|  feet,  what  is  the  work  done  per  stroke  ? 

Solution : 
W  =  area  of  diagram  X  scale  of  pressures  X  scale  of  volumes  X  144. 

=  6.3  X  30  X  1.99985  X  144  =  54,428  foot-pounds. 
W  =  mean  ordinate  X   scale  of  pressures   X    area  of  piston   X 

length  of  stroke, 
=*  2.1  X  30  X  314.159  X  2f  =?  54,428  foot-pounds. 


INDICATORS. 


Thus  we  see  that  we  get  the  same  result  by  both  rules.  The 
latter  is  the  more  common  method  because  the  mean  ordinate  is 
easily  found  and  the  scale  of  volumes  seldom  considered. 

In  our  consideration  of  Fig.  1,  steam  was  admitted  to  the 
cylinder  during  the  entire  stroke.  In  modern  engines  this  method 
is  rarely  used;  instead,  steam  is  admitted  during  part  of  the 
stroke  then  the  communication  to  the  boiler  is  cut  off,  and  the 
steam  in  the  cylinder  allowed  to  expand,  as  the  piston  moves  for- 
ward, until  it  fills  the  entire  volume  of  the  cylinder.  This  is  rep- 
resented graphically  in  Fig.  2. 

Steam  is  admitted  to  the  cylinder  until  the  piston  reaches 
the  point  2  which  repre- 
sents one-half  the  volume 
of  the  cylinder.  Then  the 
cylinder  is  half  full  of 
steam,  that  is,  it  contains 
|  =  4  cubic  feet.  The 
four  cubic  feet  of  steam 
expand  until  they  fill  the 
cylinder.  Since  there  is 
the  same  weight  of  steam 
present  at  every  point  in 
the  stroke  and  the  volume 
continued  to  increase,  the 
pressure  must  diminish. 

This  is  shown  in  Fig.  2.  The  line  B  2'  is  horizontal  because  the 
pressure  remains  constant  to  the  point  of  cut  off.  Then  the 
pressure  begins  to  fall  as  is  represented  by  the  curved  line  2'  E. 
This  curve  is  nearly  an  equilateral  hyperbola. 

From  Fig,  1  we  know  that  the  area  B  2'  2  A  is  proportional 
to  the  work  done  while  the  piston  moves  from  A  to  2  or  during 
the  first  half  of  the  stroke.  If  we  use  the  same  data  as  we  did  in 
Fig.  1,  the  work  done  must  be  one-half  the  work  done  in  the  first 

case,  or  ~Z^!?_   =    28800  foot-pounds.     Also  the  area  B  2'  2  A  is 

easily  found  since  it  is  a  rectangle.  »The  area  2'  E  0  2  is  found 
by  dividing  it  up  into  small  sections,  by  calculus  or  by  the  use  of 
a  plauimeter. 


Fi.  2. 


8 


INDICATORS. 


It  is  easily  seen  that  the  area  of  the  second  case  Fig.  2,  is 
less  than  that  of  Fig.  1.  Therefore  the  work  done  is  less  ;  but 
the  amount  of  steam  admitted  is  only  one-half  as  much  as  in  the 
first  case. 

In  the  first  case,  Fig.  1,  8  cubic  feet  of  steam  at  50  pounds 
pressure  were  admitted  per  stroke  and  the  work  done  was  found 
to  be  57600  foot-pounds.  In  the  second  case  only  half  as  much 

£fr  £»  A  A 


steam  is  admitted  and  the  work  done  is 


the     amount 


represented  by  the  area  2f  E  C  2.     Thus  we  see  that  there  is  a 
considerable  gain  by  expanding  the  steam. 

Watt's  Diagram  of  Work.     Fig.   8   illustrates  the  method 
adopted  by  James  Watt  to  show  the  action  of  steam  in  the  cylin- 


der.  The  horizontal  line  A  C  called  the  abscissa  represents  the 
length  of  the  stroke  and  is  divided  into  ten  equal  parts.  The  ver- 
tical line  A  B  called  the  ordinate  indicates  the  pressure  of  steam. 

When  the  piston  has  moved  to  the  point  E  steam  is  cut  off, 
that  is,  a  volume  of  steam  equal  to  J  the  volume  of  the  cylinder 
expands  until  it  fills  the  entire  cylinder.  The  area  may  be  found 
by  adding  the  several  pressures  (shown  by  the  dotted  lines),  divid- 
ing by  the  number  of  divisions,  and  multiplying  by  the  length. 

If  by  some  arrangement  of  steam  tight  pistons  working  in  cyl- 
inders and  having  pencils  fastened  to  them,  \ve  could  get  a  dia- 


INDICATORS. 


9 


gram  like  that  shown  in  Fig.  3  it  would  be  of  great  use  but  too 
large  for  convenience. 

To  obtain  the  same  diagram  on  a  small  scale  an  indicator  is 
used.  The  value  of  such  a  diagram  has  already  been  shown  when 
finding  the  work  done  in  the  cylinder.  The  indicator  has  enabled 
engineers  to  bring  the  engine  of  today  to  its  present  state  of  excel- 
lence. A  correct  idea  of  the  action  of  steam  in  the  cylinder  can 
be  obtained  only  by  means  of  an  indicator.  It  shows  whether  or 
not  the  valves  are  set  properly  and  how  the  condenser  is  working. 
It  also  shows  the  engineer  which  end  of  the  cylinder  is  doing  the 
most  work.  By  comparing  the  expansion  iine  with  an  equilateral 
hyperbola,  with  a  curve  of  constant  steam 
weight,  or  with  an  adiabatic  curve  for 
steam,  the  cylinder  condensation  is 
calculated. 

James  Watt  was  the  first  to  see  the 
need  of  accurate  knowledge  of  the  action  of 
steam  in  the  cylinder.  He  invented  the  in- 
dicator. The  improved  form  consisted  of  a 
steam  cylinder  S,  about  one  inch  in  diam- 
eter and  six  inches  long,  in  which  a  solid 
piston  P,  is  accurately  fitted.  A  spiral 
spring  A,  is  attached  to  this  piston,  and 
controls  the  motion  of  a  pencil  a,  which 
is  also  attached  to  the  piston.  This 
pencil  can  operate  on  a  sheet  of  paper 
fastened  to  a  sliding  board,  B.  This  board 
moves  back  and  forth  by  means  of  a  weight  at  one  end  and  a  cord 
at  the  other  which  is  connected  to  some  reciprocating  part  of  the 
engine.  The  indicator  cylinder  S,  may  be  put  in  communication 
with  the  engine  cylinder  by  means  of  the  cock  C.  With  this 
instrument  a  complete  diagram  can  be  taken. 

Watt's  first  indicator  had  no  lateral  motion,  therefore  all  it 
showed  was  the  pressure  of  steam  in  the  cylinder  and  the  perfec- 
tion of  the  vacuum. 

INDICATORS. 

The  diagram,  or  card  as  it  is  often  called,  obtained  by  the  use 
of  an  indicator  is  the  result  of  two  motions.  The  horizontal  move 


«'.  4. 


10  INDICATORS. 


ment  of  the  paper  corresponds  exactly  to  the  movement  of  the 
piston,  and  the  vertical  movement  of  the  pencil  is  an  exact  ratio 
to  that  of  the  pressure  of  steam  in  the  cylinder.  The  diagram 
represents  by  its  length  the  stroke  of  the  engine  and  by  its  height 
the  steam  pressure  on  the  piston  at  the  corresponding  point  of  the 
stroke.  The  diagram  shows  the  action  of  steam  on  one  side  of 
the  piston  only ;  to  obtain  the  same  information  in  regard  to  the 
other  side  it  is  necessary  to  take  another  diagram  from  the  other 
end  of  the  cylinder. 

The  essential  features  of  an  indicator  are  found  in  the  instru- 
ment invented  by  James  Watt.  Since  his  time,  however,  the 
many  improvements  have  made  the  indicator  light,  compact,  dura- 
ble, and  accurate.  Watt's  diagram  was  traced  on  paper  stretched 
on  a  sliding  board  but  now  a  revolving  drum  is  used.  The  height 
also  of  Watt's  diagram  was  equal  to  the  movement  of  the  spring, 
and  the  pencil  arrangement  was  a  simple  contrivance.  In  the 
indicators  of  the  present  day,  the  spring  has  a  slight  movement, 
the  height  of  the  card  being  obtained  by  a  multiplying  arrange- 
ment of  .levers.  This  method  requires  a  parallel  motion  to  obtain 
accuracy  in  the  vertical  lines ;  for  if  a  lever  is  pivoted  at  one  end 
and  power  applied  near  the  pivot  the  lever  tends  to  rise  and  the 
free  end  will  describe  an  arc  of  a  circle,  not  a  straight  vertical 
line. 

THE  THOHPSON   INDICATOR. 

Two  views  of  the  American  Thompson  Indicator,  the  outside 
and  the  inside,  are  shown  in  Figs.  5  and  6.  The  form  of  spring 
is  shown  in  Fig.  7.  The  indicator  consists  of  a  cylinder  in  which 
a  piston  is  fitted,  a  spring,  multiplying  lever  and  parallel  motion 
for  the  pencil  and  a  cylinder  or  drum  for  the  paper.  The  piston, 
which  is  .798  inch  in  diameter  =  |  square  inch  in  area,  is  fitted 
accurately  to  the  cylinder  and  has  a  travel  of  about  one-half  inch. 
When  the  pressure  of  steam  forces  the  piston  upward  it  com- 
presses the  spring  above  it ;  the  amount  of  compression  varies  with 
the  strength  of  the  spring.  The  rise  of  the  piston  causes  the 
pencil  to  rise  because  of  the  system  of  levers.  The  cylinder  to 
which  the  paper  is  attached  rotates  by  means  of  a  cord  which  is 
fctened  to  some  part  of  the  ongine,  the  crosshead  for  example. 


INDICATORS. 


II 


Fig.  5- 


While  the  drum  revolves  and  the  steam  pressure  forces  the  piston 

to  rise,  the  pencil,  lightly  touching  the  paper,  describes  the  diagram. 
The  parallel  movement  of  this  indicator  is  obtained  by  a  link 

attached  directly  to  the  lever.     It  is 

so  constructed  that  there  is  but  little 

lost  motion,  hardly  any  friction  and 

no  appreciable  error  within  the  limited 

movement  of  the  pencil. 

The   paper  cylinder  is   so  con- 
structed that  the  tension  of  the  coiled 

spring  within  the  drum  may  be  altered 

for  different  speeds  of  the  engine.    By 

this  means  the  cord  can  be  kept  taut 

with  little  trouble.     The  cord  is  led 

through  a  hold  and  kept  in  contact 

with    the    scored    wheel    by    another 

small  one.     By  this  means  the  cord 

can  be  run  to  any  angle.     It  is  con- 
venient to  have  the  cards  of  about  the  same  size.     If  we  used  a 

spring  of  such  a  tension  that  the 
pencil  would  move  one  inch  for 
every  00  pounds  pressure,  and  there 
was  but  30  pounds  pressure  in  the 
engine  cylinder,  the  diagram  would 
be  but  one-half  inch~high.  This 
diagram  would  be  too  small  for 
accurate  work.  For  this  reason 
indicators  are  provided  with  sets  of 
springs  of  varying  stiffness  which 
may  be  used  according  to  the  steam 
pressure.  If  a  spring  is  of  such 
stiffness  that  the  pencil  moves  1 
inch  for  every  50  pounds  pressure 
it  is  called  a  40  pound  spring. 
Others  are  called  10  pound,  20 
pound,  30  pound,  etc.,  springs. 
To  Change  the  Springs.  In  selecting  the  spring  for  a  given 

pressure,  care  should  be  taken  that  it  will  easily  stand  that  press- 


Fig.  6. 


INDICATORS. 


ure.     A  safe  rule  to  follow :  multiply  the  scale  of  the  spring  by 
^  and  subtract  15  for  the  vacuum. 

For  example :  The  maximum  pressure  for  a  50  pound  spring 
is  110  pounds,  because  (50  X  2|)  —  15  .=  125  --  15  =  110. 
Springs  are  made  in  the  following  scale:  8, 10,  12, 16,  20,  24,  30, 
32,  40,  48,  50,  56,  60,  64,  80,  100.  For  pressures  from  70  to  90 
pounds  a  40*  pound  spring  should  be  used,  as  80  pounds  pressure 
on  a  40  pound  spring  will  raise  the  pencil  2  inches,  and  this  is  a 
good  height  for  the  diagram. 

If  very  high  pressures  are  to  be  indicated,  an  extra  piston, 
having  an  area  of  J  square  inch,  is  used.  This 
doubles  the  allowable  pressure  on  the  spring.  For 
instance,  if  a  spring  can  be  used  for  110  pounds  press- 
ure when  the  piston  is  |  inch  in  area,  it  can  be  used 
for  220  pounds  pressure  if  the  ^  inch  piston  is  used. 

When  the  spring  has  been  selected  it  is  placed 
in  the  indicator.  First  unscrew  the  milled  nut  at 
the  top  of  the  steam  cylinder  and  take  out  the  piston 
with  arm  and  connections.  The  pencil  lever  and 
piston  are  disconnected  by  unscrewing  the  small- 
headed  screw  which  connects  them.  If  a  spring  is 
connected  to  the  piston  it  should  be  removed,  the 
selected  one  substituted  and  the  indicator  put 
together.  The  spring  should  always  be  firmly 
screwed  to  the  shoulder  or  the  pencil  will  not 
properly  indicate  the  pressure. 

Care  of  the  Indicator.  Before  attaching  the  indicator  to  the 
engine  cylinder  it  should  be  taken  apart,  cleaned  and  oiled.  If 
each  part  works  freely  and  smoothly  the  spring  may  be  put  in  and 
the  indicator  put  together.  After  connecting  to  the  cylinder, 
admit  steam  to  it,  but  do  not  take  cards  until  it  is  thoroughly 
warmed  and  blows  dry  steam  through  the  relief.  It  is  not  neces- 
sary to  use  lead  in  connecting  as  it  is  likely  to  get  into  the  indi- 
cator. After  using,  take  the  indicator  apart,  clean  and  oil.  Only 
porpoise  or  fine  watch  oil  should  be  used. 

THE  CROSBY  INDICATOR. 

The  internal  arrangement  of  the  parts  of  the  Crosby  Indicator 
*  NOTE  :  In  practice  it  is  better  to  use  a  somewhat  stiffer  spring. 


Fig.  7. 


INDICATORS. 


is  shown  in  Fig.  8.  The  piston,  8,  is  formed  with  shallow  channels 
on  its  outer  surface  to  retain  oil  which  prevents  leakage  and 
lubricates  the  piston. 

The  socket  in  the  center  of  the  piston  is  supported  by  a  cen- 
tral web  and*  projects  both  upward  and  downward.     The  upper 


Fig.  8. 

portion  is  threaded  inside  to  receive  the  lower  end  of  the  piston- 
rod.  It  has  a  vertical  slot  which  allows  the  ball  bearing  on  the 
end  of  the  spring  to  drop  into  a  concave  bearing  on  the  upper  end 
of  the  piston-screw  9  which  is  screwed  into  the  lower  part  of  the 
socket. 

The  piston-rod,  10,  is  made  hollow,  with  the  lower  end 
threaded.  When  the  piston-rod  is  connected  to  the  socket,  the 
former  should  be  screwed  into  the  socket  as  far  as  it  will  go. 

The  height  of  the  atmospheric  line  on  the  diagram  depends 


14  INDICATORS. 


upon  the  amount  the  swivel  head,  11,  is  screwed  into  the  top  of 
the  piston-rod. 

A  small  projection  on  the  lower  side  of  this  cap  is  threaded 
to  screw  into  the  top  of  the  spring  and  hold  it  firmly  in  place. 
The  moving  parts  are  kept  in  line  by  this  cap.  The  pencil 
mechanism  is  supported  by  the  sleeve  3,  which  surrounds  the 
upper  part  of  the  cylinder ;  it  turns  freely  and  is  held  in  position 
by  the  cap. 

The  pencil  mechanism  is  made  as  light  as  is  consistent  with 
strength  and  stiffness.  The  pencil  moves  exactly 
parallel  to  the  piston  because  the  fulcrum  of  the 
mechanism  and  the  point  of  attachment  to  the 
piston-rod  are  always  in  a  straight  line.  The 
pencil  point  moves  six  times  as  far  as  the  piston 
because  of  the  multiplying  levers. 

The  drum  24,  is  one  and  one-half  inches  in 
diameter,  and  is  rewound  when  the  string  is  pulled, 
by  a  short  spiral  spring  31. 

The  piston  spring  is  made  of  a  single  piece 
of  steel  wire  wound  from  the  middle  into  a 
double  coil.  The  ends  are  screwed  into  a  brass  head 
having  four  radial  wings.  At  the  bottom  of  the 
spring  a  small  steel  bead  is  firmly  attached  to  the  wire.  This 
forms  a  ball  and  socket  joint  with  the  lower  end  of  the  piston-rod. 
This  joint  is  light  and  allows  the  spring  to  yield  to  pressure  from 
any  direction.  These  springs  are  made  in  the  following  scale : 
8, 12,  16,  20,  24,  30,  40,  50,  60,  80,  100,  120,  150,  and  180. 

To  Insert  the  Spring.  First  unscrew  the  cap  2,  then  lift  the 
connected  parts  free  from  the  cylinder  by  means  of  the  sleeve. 
The  hollow  wrench  should  be  held  in  an  inverted  position  arid 
the  piston-rod  inserted  until  the  hexagonal  'part  engages  the 
wrench.  Then,  having  the  spring  shown  in  Fig.  9  inverted, 
insert  the  combined  wrench  and  piston-rod  until  the  steel  bead 
and  the  end  of  the  spring  rests  in  the  concave  seat.  Now  invert 
the  piston  and  pass  the  transverse  wire  at  the  bottom  of  the  spring 
through  the  slot  until  the  threads  at  the  bottom  of  the  piston-rod 
engage  those  inside  the  socket  of  the  piston.  With  the  wrench 
screw  it  in  as  far  as  it  will  go. 


INDICATORS.  15 


The  piston  screw  should  be  loosened  slightly  before  the 
piston-rod  is  screwed  in,  and  afterward  set  up  against  the  bead 
lightly  to  prevent  lost  motion.  Then  with  the  sleeve  and  cap 
upright,  engage  the  threads  of  the  swivel  bead  with  those  inside 
the  piston-rod  and  screw  it  up  until  the  lower  projection  of  the 
cap  engages  the  threads  inside  the  spring  top;  continue  the  process 
until  the  spring  is  screwed  up  firmly  against  the  cap.  Holding 
only  by  the  sleeve  3,  turn  the  piston  and  the  connections  until 
the  top  of  the  piston-rod  is  flush  with  the  shoulder  on  the  swivel 
head. 

Now  that  the  piston  and  all  the  connections  are  in  their 
places,  the  whole  may  be  inserted  in  the  cylinder  and  the  cap 
screwed  down,  which  will  fix  all  parts  in  their  proper  places. 
If  there  is  a  spring  in  the  cylinder  first  detach  by  reversing  the 
above  process. 

THE   TABOR   INDICATOR. 

The  Tabor  Indicator  is  shown  in  Fig.  10.  It  is  used  exten- 
sively in  the  navy.  The  principle  of  action  and  details  of  con- 
struction are  similar  to  the  indicators  already  described  ;  the  chief 
peculiarity  being  the  means  employed  to  obtain  a  straight  line 
movement  for  the  pencil.  Inside  the  steam  cylinder  is  a  lining  in 
which  the  piston  moves.  This  lining  can  expand  when  heated. 
In  the  side  of  the  cylinder  small  holes  allow  any  steam  which  may 
leak  by  the  piston  to  escape.  The  piston-roJ.  is  connected  at  one 
end  to  the  piston  by  means  of  a  ball  and  socket  joint ;  the  other 
end  is  connected  to  the  pencil  mechanism. 

The  pencil  mechanism  consists  of  three  pieces,  the  pencil 
lever,  the  back  link,  and  the  piston-rod  link.  The  two  links  are 
parallel  for  every  position  of  the  pencil.  Thus  the  lower  pivots 
of  these  links  and  the  pencil  point  are  always  in  the  same  straight 
line.  The  straight  line  movement  of  the  pencil  is  obtained  by 
means  of  a  curved  slot  in  a  stationary  plate.  The  pencil  bar  is 
provided  with  a  roller  which  is  fitted  in  such  a  manner  that  it  can 
roll  from  one  end  of  the  slot  to  the  other.  The  curve  of  the  slot 
guides  the  pencil  bar  and  is  of  such  a  radius  that  the  pencil  is 
caused  to  move  in  a  straight  line.  The  curve  compensates  for  the 
tendency  of  the  pencil  point  to  move  in  an  arc  of  a  circle.  The 


Id 


INDICATORS. 


pressure  of  the  pencil  on  the  paper  is  regulated  by  a  screw  which 
strikes  a  stop  plate  attached  to  the  frame.  The  end  of  the  pencil 
bar  is  formed  for  either  a  pencil  lead  or  a  metallic  marking  point. 

The  drum  for  the  paper  is  made  similar  to  those  of  other 
indicators.  The  backward  movement  is  obtained  by  a  flat  spiral 
spring  placed  under  the 
drum.  The  tension  of 
this  spring  is  altered  by 
loosening  a  thumbscrew, 
lifting  the  carriage,  and 
winding  or  unwinding. 
A  simple  pulley  guides 
the  driving  cord  in  any 
direction. 

The  indicator  is  at- 
tached by  a  coupling 
having  a  single  thread. 

The  springs  of  the 
Tabor  indicator  are  of 
the  duplex  type,  that  is, 
they  are  made  of  two 
spiral  coils  of  wire.  A 
50  pound  spring  is 
shown  in  Fig.  11.  The 
wire  terminates  in  fit- 
tings at  each  end.  The  spring  is  attached  to  the  upper  side  of  the 
piston  by  means  of  threads  cut  on  the  inside  of  the  fitting  and  on  a 
projection  on  the  piston.  The  top  of  the  spring  is  attached  to  the 
under  side  of  the  cover  in  a  similar  manner.  The  springs  are 
made  in  the  following  scales,  8,  10,  12,  16,  20,  24,  30,  32,  40,  48, 
50,  60,  64,  80,  and  100  pounds.  The  maximum  safe  steam  pres- 
sures (absolute)  to  which  these  springs  may  be  subjected  are 
respectively,  10,  15,  20,  24,  40,  48,  70,  75,  95,  112, 120,  140, 152, 
180,  and  200  pounds. 

Change  of  Location  of  Atmospheric  Line.  Unscrew  the  cap 
and  lift  the  sleeve  and  connections  from  the  cylinder.  Then  turn 
the  piston  to  the  left  or  right  according  as  the  pencil  is  to  be 
raised  or  lowered.  One  revolution  causes  the  pencil  to  rise  J 
inch. 


Fig.  10. 


INDICATORS.  17 


Care  of  the  Indicator.  Before  attaching  the  indicator  to  the 
engine,  steam  should  be  blown  through  the  pipes  and  cocks  so 
that  all  particles  of  dust  may  be  removed.  After  using,  the  indi- 
cator should  be  carefully  wiped  and  oiled.  The  cylinder  cap 
should  be  unscrewed  and  all  the  parts  connected  to  the  piston 
removed.  The  piston  spring  and  piston-rod  should  be  detached, 
carefully  wiped  dry,  and  then  oiled.  The  inside  of  the  cylinder 
also  should  be  oiled.  Then  the  piston  and  piston-rod  should  be 
placed  in  the  cylinder  and  the  spring  placed  in  the  box.  If  the 
indicator  has  not  been  used  for  some  time  the  oil  may  have  become 
gummed.  It  may  be  easily  cleaned  by  wiping  with 
{i  cloth  saturated  with  naphtha  or  benzine.  It  must 
be  oiled  again  before  using.  A  good  test  that  the 
indicator  is  in  proper  working  order  is  to  detach  the 
spring  and  after  replacing  the  piston  and  piston-rod, 
raise  the  pencil  to  the  highest  point.  When  allowed 
to  fall  it  should  descend  to  the  lowest  point  freely. 
The  pencil  should  always  have  a  smooth  fine  point. 
To  Attach  the  Indicator  to  the  Engine.  Usually 
all  first-class  engines  are  prepared  for  the  indicator 
before  leaving  the  factory.  Holes  are  drilled  and 
tapped  in  the  cylinder  and  have  plugs  screwed  in  them.  These 
plugs  are  easily  removed  and  the  indicator  connections  screwed  in. 
When  this  is  not  the  case,  any  engineer  can  perform  the 
work.  Before  drilling  the  holes,  in  the  cylinder,  the  heads  should 
be  removed  so  that  the  exact  positions  of  the  pistons  and  the  size 
of  the  ports  and  passages  may  be  known.  Also  with  the  heads  off 
all  chips  and  particles  of  dirt  from  drilling  may  be  easily  removed. 
If  it  is  impossible  to  remove  the  heads,  a  little  steam  admitted  to 
the  cylinder  just  before  the  drilling  »  completed  will  blow  the 
chips  out. 

Each  end  should  be  drilled  and  tapped  for  a  one-half  inch 
pipe  thread.  The  holes  must  be  drilled  into  the  clearance  space, 
so  that  the  piston  at  the  ends  of  the  stroke  will  not  cover  them. 
They  should  also  be  placed  so  that  currents  of  steam  will  not  reach 
them.  Before  deciding  just  the  points  at  which  to  drill  the  holes, 
it  is  well  to  consider  every  plan  of  indicating  the  engine.  The 
type  of  engine,  the  position  of  the  steam  chest,  the  kind  of  cross- 


18 


INDICATORS. 


head,  and  the  position  of  the  eccentric  and  its  connections  should 
all  be  considered,  as  well  as  the  most  convenient  place  in  the 
engine  room.  The  holes .  should  not  be  drilled  until  the  plan 
shows  the  proper  connections  with  the  reducing  motion,  conven- 
ient access  and  free  passage  of  steam  to  the  indicator. 

When  the  plan  has  been  adopted,  the  engine  should  be  placed 
on  dead  center,  to  determine  the  clearance.  The  holes  should  be 
drilled  into  the  middle  of  the  clearance  space. 

In  common  practice  for  horizontal  engines  the  holes  are 
drilled  in  the  side  of  the  cylinder  at  each  end.  Short  half  inch 
pipes  with  quarter  upward  bends  into  which  the  indicator  coils 
may  be  screwed  are  inserted  in  these  holes. 

It  may  be  more  convenient  to  drill  and  tap  into  the  top  of 
the  cylinder  and  attach  the  indicators  directly. 

For  vertical  engines,  the  upper  head  or  cover  and  the  side  of 
the  cylinder  are  often  drilled  and  tapped  for  the  upper  and  lower 
indicators  respectively.  It  is  preferable  to  connect  the  indicators 


Fig.  12. 

to  the  sides  because  less  pipe  and  fittings  are  required  and  better 
results  obtained. 

If  only  one  indicator  is  to  be  used  for  both  ends  of  the  cyl- 
inder, it  may  be  connected  by  side  pipes  and  a  three  way  cock. 
By  this  method  both  diagrams  are  taken  on  the  same  card  and 
with  the  loss  of  but  one  revolution.  Fig.  12  shows  the  section  of 
a  three  way  cock. 

Reducing  notion.  As  we  have  already  seen  the  length  of 
the  card  represents  the  travel  of  the  piston.  As  the  length  of 


INDICATORS. 


19 


card  is  obtained  by  the  rotation  of  the  drum  the  motion  of  the 
drum  must  be  taken  from  some  part  of  the  engine  which  has  a 
motion  coinciding  with  that  of  the  piston.  The  crosshead  is  the 
most  common,  reliable  and  convenient  part.  The  length  of  the 
card  is  much  less  than  the  travel  of  the  piston  since  the  stroke  is 
longer  than  ^he  circumference 
of  the  drum  so  that  the  move- 
ment of  the  crosshead  must 
be  reduced  to  the  length  of 
the  diagram. 

There  are  several  devices 
employed  to  obtain  this  re- 
duced motion. 

A  simple  form  of  reduc- 
ing motion,  called  the  panto- 
graph, is  shown  in  Fig.  13. 
Four  links,  a,  5,  <?,  d,  are 
joined  in  the  form  of  a  par- 
allelogram. One  link,  «,  is 
prolonged  and  pivoted  at  the 
crosshead  C.  The  point  where 
b  and  c  join  is  pivoted  at  the 
fixed  point  E.  The  cord  is 
fastened  at  D  on  the  link  d. 


Fig.  13. 


The  point  D  must  be  in  the  straight  line  connecting  E  and  C. 
Then  letting  A  B  represent  the  stroke  and  h  the  length  of  the 
indicator  diagram,  we  have 

A  B  :  h  =  E  C  :  E  D,  from  which 


ED  = 


h  X  EC 
AB 


Another  form  of  pantograph  is  shown  in  Fig.  14.  It  is  placed 
horizontally  with  the  pivot,  B,  resting  on  a  support  opposite  the 
crosshead  when  in  mid-position.  The  pivot  A,  is  attached  to  the 
crosshead  ;  usually  by  having  the  stud  A  inserted  in  a  hole  drilled 
in  the  crosshead.  If  the  pivot  B  is  adjusted  to  the  proper  height 
and  at  the  right  distance  from  the  crosshead,  the  cord  from  the 
indicator  may  be  attached  to  the  pin  E  without  any  pulleys.  The 


INDICATORS. 


length  of  the  diagram  is  varied  by  adjusting  the  movable  bar  C  D; 
the  pin  E  must  be  in  the  straight  line  from  A  to  B. 

The  pantograph  is  likely  to  become  shaky  and  loose  on 
account  of  its  many  joints.  If  well  made  it  gives  perfect  motion. 

The  reducing  motion  shown  in  Fig.  15,  called  the  Brumbo 
Pulley,  is  easily  and  quickly  made  and  can  be  used  on  almost  any 
engine.  The  wooden  rod  A  is  usually  about  twice  as  long  as  the 
stroke.  It  is  pivoted  by  a  bolt  or  screw  at  B,  a  fixed  point.  At  the 
lower  end  it  is  connected  by  the  wooden  link,  C,  to  the  crosshead. 


Fig.  H 

This  link  C  is  usually  about  one-half  the  length  of  the  stroke.  The 
sector  S  may  be  made  either  of  wood  or  metal.  It  should  have  a 
groove  in  the  circular  edge  for  the  cord,  and  is  made  fast  to  the  upper 
end  of  the  lever  A.  Its  center  should  coincide  with  that  of  the 
pivot  B.  The  length  of  the  radius  of  the  sector  may  be  found  as 
follows.  Divide  the  length  of  the  lever  by  the  length  of  the 
stroke,  multiply  the  result  by  the  length  of  the  desired  diagram 
and  the  product  will  be  the  radius  of  the  sector.  For  instance,  if 
the  lever  is  60  inches  long  and  the  stroke  30  inches  and  we  wish 
the  diagram  to  be  3  inches  long,  the  sector  should  be  6  inches  in 
radius  for, 


To  avoid  the  use  of  guide  pulleys  the  lever  should  be  hung 
so  that  it  will  swing  in  a  vertical  plane  parallel  with  the  guides  and 
in  line  with  the  indicator.  When  the  crosshead  is  at  mid-stroke, 


INDICATORS. 


21 


the  lever  must  be  vertical  and  the  point  D  must  be  below  the  axis 
of  the  cylinder  because  it  comes  above  the  line  at  the  ends  of  the 
stroke. 

The  reducing  lever  used  for  large  quick  running  engines  is 
shown  in  Fig.  16.  The  rod  A  is  made  of  pine  wood,  tapering 
toward  the  lower  point  and  about  one  inch  in  thickness.  The 
length  is  about  one  and  one-half  times  the  length  of  the  stroke. 
It  is  suspended  by  a  bolt  o:  screw  from  some  fixed  point  above  the 
engine,  and  should  swing  edgewise  and  parallel  to  the  guides  of 
the  crosshead.  The  steel  stud  at  the  bottom  of  the  rod  has  a  T 
shaped  slot  in  an  iron  plate  which  is  attached  firmly  to  the  cross- 
head.  The  slot  should  be  long  enough  to  retain  the  stud  when 
the  crosshead  is  at  the  end  of  the  stroke.  To  find  the  point  at 
which  the  indicator  cord  should  be  attached,  divide  the  length  of 


Fig,  15. 


Fig.  16. 


the  lever  by  the  length  of  the  stroke  of  the  piston  and  multiply 
the  quotient  by  the  length  of  the  desired  diagram.  The  product 
isvtlie  distance  of  the  point  from  the  pivot  at  the  top  of  the  lever. 
Example.  A  lever  is  45  inches  long,  the  piston  stroke  30 
inches  and  the  diagram  to  be  3J  inches  long.  At  what  distance 
from  the  pivot  should  the  indicator  cord  be  attached  ? 


Having  placed  the  indicator  in  position   and   obtained   the 


22  INDICATORS. 


reducing  motion,  the  length  of  the  cord  must  be  so  adjusted  that 
the  drum  will  not  strike  the  stops  at  either  end  of  the  stroke. 

For  convenience  an  approximate  length  of  cord  is  first  found 
and  the  cord  cut  in  two  parts,  one  attached  to  the  reducing 
motion  ;  the  other  to  the  indicator.  A  hook  should  be  fastened  to 
the  froe  end  of  the  piece  attached  to  the  indicator.  A  loop  is 
made  in  the  free  end  of  the  piece  from  the  reducing  motion. 
The  hook  is  then  attached  to  the  irop  and  the  extra  length 
of  cord  taken  up  by  tying  knots.  Another  method  for  adjust- 
ing the  length  of  the  cord  is  the  arrangement  shown  in  Fig. 
17.  The  hook  A  is  attached  to  the  indicator  cord.  The  cord  B 
from  the  reducing  motion  passes  through  the  holes  in  the  plate  P 
as  shown.  To  adjust  the  length  of  the  cord  it  is  slacked  at  the 
point  B  and  the  plate  slipped  along  the  cord. 


Fig.  17. 

To  Take  Indicator  Diagrams.  The  indicator  should  be  in 
good  working  order  before  attaching  to  the  cylinder ;  it  should  be 
clean,  well  oiled,  and  the  levers  and  springs  should  work  smoothly. 
The  pencil  point  should  be  sharp  and  the  pressure  adjusted  to 
make  a  distinct  fine  line. 

The  spring  should  be  selected  that  will  give  a  diagram  1^ 
to  2  inches  in  height.  If  the  spring  chosen  is  too  light,  the  lines 
are  likely  to  be  wavy  from  the  vibration  of  the  pencil  levers. 

When  the  indicator  is  in  position  a  satisfactory  reducing 
motion  obtained  and  the  cord  adjusted,  the  paper  should  be 
wrapped  smoothly  around  the  drum.  The  edges  projecting  over 
the  clips  should  be  folded  back  so  that  they  will  not  touch  the 
pencil  lever.  Before  taking  the  card,  allow  the  steam  to  enter 
the  indicator  and  move  the  piston  up  and  down  until  the  parts 
have  become  thoroughly  warmed.  Then  pass  the  pencil  against 
the  paper  long  enough  to  take  the  diagram.  Some  engineers 
allow  the  pencil  to  remain  in  contact  with  the  paper  during  but 
one  revolution  of  the  engine ;  others  trace  the  diagram  two  or 


INDICATORS.  23 


more  times.  After  the  diagram  is  taken  the  cock  is  shut  and 
without  unhooking  the  cord  the  pencil  is  again  pressed  to  the 
paper  to  take  the  atmospheric  line.  The  cord  is  then  disconnected 
and  the  card  removed  from  the  drum.  The  scale  of  spring,  the 
dimensions  and  speed  of  the  engine,  the  date,  and  all  useful  partic- 
ulars are  written  on  the  cards. 

If  one  indicator  is  used  for  both  ends  the  three  way  cock 
shown  in  Fig.  12  is  opened  to  admit  steam  from  one  end,  the 
diagram  taken  ;  then  opened  for  the  other  end  and  that  diagram 
taken.  Then  the  steam  is  shut  off  from  both  ends  and  the  atmos- 
pheric line  taken. 

As  has  been  said  before,  the  indicator  is  of  great  importance 
to  the  engineer.  It  is  used  to  find  the  indicated  horse-power  of 
the  engine,  and  by  comparison  of  the  indicated  horse-power  with 
the  brake  horse-power,  the  mechanical  efficiency  is  obtained.  The 
indicator  card  shows  several  other  things  ;  the  time  and  manner 
of  the  four  events  of  the  stroke,  namely,  the  admission,  cut-off, 
release,  and  compression.  These  four  events  make  up  what  is 
called  the  steam  distribution.  It  shows  faults  in  the  setting  and 
working  of  the  valves. 

We  have  seen  how  the  indicator  diagram  represents  the  net 
work  done  on  the  piston  in  one  stroke. 

Work  is  equal  to  pressure  multiplied  by  the  distance  through 
which  it  acts.  The  distance  is  the  length  of  stroke  multiplied  by 
the  number  of  strokes  per  minute.  The  pressure  is  the  average 
net  pressure  acting  on  the  piston  during  the  stroke.  This  average 
net  pressure  is  called  the  mean  effective  pressure.  If  we  know 
from  the  indicator  card  the  mean  effective  pressure  per  square 
inch,  we  can  find  the  total  pressure  by  multiplying  it  by  the  area 
of  the  piston  in  square  inches. 

The  distance  per  minute  is  equal  to  the  length  of  stroke 
multiplied  by  the  number  of  strokes. 

Let  1*  —  mean  effective  pressure  in  pounds  per  square  inch. 
A  —  area  of  piston  in  square  inches. 
L  —  length  of  stroke  in  feet. 
N  —  number  of  strokes  per  minute. 
Then  the  work  done  per  minute, 

W  =  P  L  AK. 


24  INDICATORS. 


Since  one  horse-power  is  the  rate  of  doing  work  when  33,000 
foot-pounds  of  work  are  done  per  minute  the  indicated  horse-power 
of  an  engine  is  obtained  by  means  of  the  formula, 

H  P  =   PL AN 
33,000 

The  length  of  the  stroke,  the  area  of  the  piston  and  the  num- 
ber of  strokes  are  easily  found.  Then  all  that  remains  to  be 
determined  before  the  horse-power  is  calculated  is  the  value  of  P. 
or  the  mean  effective  pressure. 

Suppose  one  side  of  the  piston  is  in  communication  with  the 
boiler  during  the  entire  stroke,  the  mean  pressure  is  then  the  boiler 
pressure.  But  if  the  supply  of  steam  is  cut  off  before  the  stroke 
is  completed,  the  mean  pressure  will  not  equal  the  boiler  pressure. 
In  Fig.  1  we  saw  that  the  area  of  the  shaded  portion  was  equal  to 
the  length  multiplied  by  the  height.  The  area  of  a  figure  of  any 
shape  can  be  reduced  to  that  of  a  rectangle  having  a  length  equal 
to  the  extreme  length  of  the  figure.  Then  whenever  we  know  the 
area  and  length,  we  can  find  the  height  or  mean  height  by  dividing 
the  area  by  the  length. 

Suppose  a  diagram  like  that  shown  in  Fig.  2  has  an  area  of 
5^  square  inches  and  its  extreme  length  is  3i  inches  ;  then  the 
height  is  5.25  divided  by  3.5  =  1.5  inches.  Then  with  any  indi- 
cator card  the  area  of  the  diagram  is  equal  to  the  area  of  a 
rectangle,  the  length  of  which  is  known  and  the  height  can  easily 
be  computed. 

Suppose  that  we  have  taken  a  card  and  know  that  the  mean 
height  is  1 J  inches.  In  order  to  find  the  horse-power  we  must 
reduce  the  1|-  inches  to  pounds  pressure.  If  we  multiply  the 
height  by  the  strength  of  the  spring  we  get  the  desired  result. 
For  instance,  if  we  had  used. a  30  pound  spring  (that  is  one  which 
causes  the  pencil  of  the  indicator  to  move  one  inch  for  every  30 
pounds  pressure  in  the  cylinder)  the  height  1|  inches  would 
equal  30  X  1^  =  45  pounds  pressure. 

Example.  An  indicator  card  has  an  area  of  1.925  square 
inches  and  is  2.2  inches  long.  If  a  60  pound  spring  is  used 
what  is  the  mean  pressure  ? 

The  mean  height  equals  -       '-  =  .875  inch  and 

2t.2t 

.875  x  60  =  52.5  pounds, 


INDICATORS.  25 


Suppose  the  engine  from  which  the  above  card  was  taken  had 
a  piston  14  inches  in  diameter  and  a  stroke  of  24  inches.  If  it 
were  running  at  150  revolutions  per  minute  what  is  its  horse- 
power ?  Assume  the  mean  effective  pressure  to  be  the  same  for 
both  sides  of  the  piston. 

H   P   =    P  L  A  N 
33,000 

_  52.5  X  2  X  153.94  X  300 

33,000 

=147.     (about) 

In  most  engines  more  work  is  done  at  one  end  of  the  cylinder 
that  at  the  other;  it  is  not  safe  then  to  assume  the  mean  effective 
pressure  of  one  side  the  same  as  that  of  the  other.  Cards  should 
be  taken  from  each  end  and  calculated  for  mean  effective  pressure 
separately,  then  averaged.  Also  the  area  of  one  side  of  the  piston 
is  greater  than  the  other  on  account  of  the  piston-rod.  The  two 
ends  may  be  figured  separately  or  the  average  area  of  the  two  sides 
of  the  piston  may  be  used  as  the  value  of  A. 

Another  method-  is  to  find  the  work  done  at  each  end  of  the 
cylinder  and  then  add  the  results.  This  enables  the  engineer  to 
know  if  his  valves  are  set  so  that  each  end  does  about  the  same 

amount  of  work. 

An  engine  has  the  following  dimensions.  The  piston  is  12 
inches  in  diameter,  the  piston-rod  is  21  inches  in  diameter  and  the 
length  of  stroke  is  34  inches.  While  running  at  92  revolutions 
cards  were  taken.  The  area  of  the  card  from  the  head  end  was 
5.36  square  inches,  that  of  the  crank  end  was  5.30  square  inches, 
and  a  40  pound  spring  was  used.  The  cards  were  3.72  inches 
long.  We  wish  to  know  what  horse-power  the  engine  developed 
and  which  end  was  doing  the  most  work. 

The  area  of  the  piston  is  113.097  square  inches  for  the  head 
end  and  113.097  —  3.5466  =  109.55  square  inches  for  the  crank 
end. 

Then  for  the  head  end, 
H  P          PLAN       .  P  X  34  X  113.097  X  92  _     893g  p 

33,000  12  X  33,000 

and  for  the  crank  end, 

H   p         PLAN       P  X  34  X  109.55  X  92         8  .    p 
33,000  12  X  33,000 


26  INDICATORS. 


The  value  of  P  is  found  from  the  cards.     Since  the  cards 
were  3.72  inches  long  and  the  area  of  the  card  from  the  head  end 

5  36 
was  5.36  square  inches  the  mean  ordinate  is  __!__ _  =  1.44  inches 

which  equals  1.44  X  40  =  57.6  pounds.  The  horse-power  would 
be, 

.8933  X  57.6  =  51.45 

For  the  crank  end  the  mean  effective  pressure  is  found  as 
before, 

=  1.425  and  1.425  X  40  =  57. 
3.72 

The  horse-power  would  be, 

.8653  X  57  =  49.32 

The  horse-power  is  evidently  the  sum  of  these  two  quantities 
or  51.45  +  49.32  =  100.77. 

The  head  end  is  doing  more  work  than  the  crank  end  but  the 
difference  is  slight  being  only, 

51.45  -  -  49.32  =  2.13  horse-power, 
or  about  2.1  per  cent,  of  the  total  power. 

Considerable  arithmetical  work  is  necessary  when  the  I.  H.  P. 
is  found  from  the  formula, 

I  H  P.  =    PLAN 
33,000 

and  the, chances  for  error  are  of  course,  great.  To  safe  time  and 
reduce  the  chance  for  error  a  table  of  engine  constants  lias  been 
prepared.  The  number  of  strokes,  or  twice  the  number  of  revo- 
lutions, multiplied  by  the  length  of  stroke  in  feet  is  called  the 

piston  speed.     Then  in  the  formula  I.  H.  P.  =  _  ,    L  N  = 

33,000 

piston  speed  in  feet  per  minute.  In  the  following  table,  the  I.  H.  P. 
of  an  engine  is  easily  computed  by  multiplying  the  constant,  cor- 
responding to  the  diameter  of  the  piston,  by  the  piston  speed  and 
by  the  M.  E.  P.  Or,  in  other  words,  the  constants  in  the  table 
equal  the  horse-power  for  an  engine  with  a  given  diameter  of 
piston  having  a  piston  speed  of  one  foot  per  minute  and  a  M.  K.  P. 
of  one  pound. 


INDICATORS. 


27 


TABLE  OF  ENGINE   CONSTANTS, 


Diam- 

! 

eter 

Even 

-H 

+  1   +i 

+  * 

+  1 

•M 

+  1 

of 

or 

or       or 

or 

or 

or 

or 

Cylin- 

Inches. 

.125. 

.25.      .375. 

.5. 

.625. 

.75. 

.875. 

der. 

1 

0000238 

.0000301 

.0000372 

.0000450 

.0000535 

.0000628 

.0000729 

.0000837 

2 

0000952 

.0001074 

.0001205 

.0001342 

.0001487 

.0001640 

.0001800 

.0001967 

3 

.0002142 

.0002324 

.0002514 

.0002711 

.0002915 

.0003127 

.0003347 

.0003574 

4 

.OUU3»08 

.0004050 

.0004299 

.0004554 

.0004819 

.0005091 

.0005370 

.0005656 

5 

.0005950 

.0006251 

.0006560 

.0006876 

.0007199 

.0007530 

.0007869 

.0008215 

6 

.0008568 

.0008929 

.0009297 

.0009672 

.0010055 

.0010445 

.0010844 

.0011249 

7 

.0011662 

.0012082 

.0012510 

.0012944 

.0013387 

.0013837 

.0014295 

.0014759 

8 

.0015232 

.0015711 

.0016198 

.0016693 

.0017195 

.0017705 

.0018222 

.0018746 

9 

.0019278 

.0019817 

.0020363 

.0020916 

.0021479 

.0022048 

.0022625 

.0023209 

10 

.0023800 

.0024398 

.0025004 

.0025618 

.0026239 

.0026867 

.0027502 

.0028147 

11 

.0028798 

.0029456 

.0030121 

.0030794 

.0031475 

.0032163 

.0032859 

.0033561 

12 

.0034272 

.0034990 

.0035714 

.0036447 

.0037187 

.0037934 

.0038690 

.0039452 

13 

.0040222 

.0040999 

.0041783 

.0042576 

.0043375 

.0044182 

.0044997 

.0045819 

14 

.0046648 

.0047484 

.0048328 

.0049181 

'  .0050039 

.0050906 

.0051780 

.0052661 

15 

.0053550 

.0054446 

.0055349 

.0056261 

.0057179 

.0058105 

.0059039 

.0059979 

16 

.0060928 

.0061884 

.0062847 

.0063817 

.0064795 

.0065780 

.0066774 

.0067774 

17 

.0068782 

.0069797 

.0070819 

.0071850 

.0072887 

.0073932 

.0074985 

.0076044 

18 

.0077112 

.0078187 

.0079268 

.0080360 

.0081452 

.0082560 

.0083672 

.0084791 

19 

.0085918 

.0087052 

.0088193 

.0089343 

.0090499 

.0091663 

.0092835 

.0094013 

20 

.0095200 

.0096393 

.0097594 

.0098803 

.0100019 

.0101243 

.0102474 

.0103712 

21 

.0104958 

.0106211 

.0107472 

.0108739 

.0110015 

.0111299 

.0112589 

.0113886 

22 

.0115192 

.0116505 

.0117825 

.0119152 

.0120487 

.0121830 

.0123179 

.0124537 

23 

.0125902 

.0127274 

.0128654 

.0130040 

.0131435 

.0132837 

.0134247 

.0135664 

24 

.0137088 

.0138519 

.0139959 

.0141405 

.0142859 

.0144321 

.0145789 

.0147266 

25 

.0148750 

.0150241 

.0151739 

.0153216 

.0154759 

.0156280 

.0157809 

.0159345 

26 

.0160888 

.0162439 

.0163997 

.0165563 

.0167135 

.0168716 

.0170304 

.0171899 

27 

.0173502 

.0175112 

.0176729 

.0178355 

.0179988 

.0181627 

.0183275 

.0184929 

28 

.0186592 

.0188262 

.0189939 

.0191624 

.0193316 

.0195015 

.0196722 

.0198436 

29 

.0200158 

.0201887 

.0203624 

.0205368 

.0207119 

.0208879 

.0210645 

.0212418 

30 

.0214200 

.0215988 

.0217785 

.0219588 

.0221399 

.0223218 

.0225044 

.0226877 

31 

.0228718 

.0230566 

.0232422 

.0234285 

.0236155 

.0238033 

.0239919 

.0241812 

32 

.0243712 

.0245619 

.0247535 

.0249457 

.0251387 

.0253325 

.0255269 

.0257222 

33 

.0259182 

.0261149 

.0263124 

.0265106 

.0267095 

.0269092 

.0271097 

.0273109 

34 

.0275128 

.0277155 

.0279189 

.0281231 

.0283279 

.0285336 

.0287399 

.0289471 

35 

.0291550 

.0293636 

.0295729 

.0297831 

.0299939 

.0302056 

.0304179 

.0306309 

36 

.0308448 

.0310594 

.0312747 

.0314908 

.0317075 

.0319251 

.0321434 

.0323624 

37 

.0325822 

.0328027 

.0330239 

.0332460 

.0334687 

.0336922 

.0339165 

.0341415 

38 

.0343672 

.0345937 

.0348209 

.0350489 

.0352775 

.0355070 

.0357372 

.0359685 

39 

.0361998 

.0364322 

.0366654 

.0368993 

.0371339 

.0373694 

.0376055 

.0378424 

40 

.0380800 

.0383184 

.0385575 

.0387973 

.0390379 

.0392793 

.0395214 

.0397642 

41 

.0400078 

.0402521 

.0404972 

.0407430 

,0409895 

.0412368 

.0414849 

.0417337 

42 

.0419832 

.0422335 

.0424845 

.0427362 

.0429887 

.0432420 

.0434959 

.0437507 

43 

.0440062 

.0442624 

.0445194 

.0447771 

.0450355 

.0452947 

.0455547 

.0458154 

44 

.0460768 

.0463389 

.0466019 

.0468655 

.0471299 

.0473951 

.0476609 

.0479276 

45 

.0481950 

.0484631 

.0487320 

.0490016 

.0492719 

.0495430 

.0498149 

.0508875 

46 

.0503608 

.0506349 

.0509097 

.0511853 

.0514615 

.0517386 

.0520164 

.0522949 

47 

.0525742 

.0528542 

.0531349 

.0534165 

.0536988 

.0539818 

.0542655 

.0545499 

48 

.0548352 

.0551212 

.0554079 

.0556953 

.0559835 

.0562725 

.0565622 

.0568526 

49 

.0571438 

.0574357 

.0577284 

.0580218 

.0583159 

.0586109 

.0589065 

.0592029 

50 

.0595000 

.0597979 

.0600365 

.0603959 

.0606959 

.0609969 

.0612984 

.0616007 

51 

.0619038 

.0622076 

.0625122 

.0628175 

.0632235 

.0634304 

.0637379 

.0640462 

52 

.0643552 

.0646649 

.0649753 

.OR52867 

.0655987 

.0659115 

.0662250 

.0665392 

53 

.0668542 

.0671699 

.0674864 

.0678036 

.0681215 

.0684402 

.0687597 

.0690799 

54 

.0694008 

.0697225 

.0700449 

.0703681 

.0705293 

.0710166 

.0713419 

.0716681 

55 

.0719950 

.0724226 

.0726510 

.0729801 

.0733099 

.0736406 

.0739719 

.0743039 

56 

.0746368 

.0749704 

.0753047 

.0756398 

.0759755 

.0763120 

.0766494 

.0769874 

57 

.0773262 

.0776657 

.0780060 

.0783476 

0786887 

.0790312 

.0793745 

.0797185 

58 

.0800632 

.0804087 

.0807549 

.0811019 

.0814495 

.0817980 

.0821472 

.0824971 

59 

.0828478 

.0831992 

.0835514 

.0839043 

.0842579 

.0846123 

.0849675 

.0853234 

60 

.0856800 

.0860374 

.0863955 

.0867543 

.0871139 

.0874743 

.0878354 

.0881973 

INDICATORS. 


To  Use  the  Table.  If  the  diameter  of  the  piston  is  an  even 
number,  the  constant  is  found  in  the  second  column ;  if  it  contains 
a  fraction  the  constant  is  found  by  following  the  column  horizon- 
tally until  the  required  fraction  is  reached.  The  constant  multi- 
plied by  the  piston  speed  in  feet  per  minute  ai  cl  by  the  M.  E.  P. 
in  pounds  per  square  inch  gives  the  I.  H.  P. 

Example.  An  engine  runs  at  75  revolutions.  The  stroke  is 
4  feet;  if  the  M.  E.  P.  is  48  pounds  and  the  piston  27f  inches  in 
diameter  what  is  the  I.  H.  P. 

From  the  table  the  constant  for  a  piston  27 f  inches  in  diam- 
eter is  .0178355.  The  piston  speed  is  150  X  4  =  600  feet  per 
minute.  Then  the  I.  H.  P.  is, 

.0178355  X  600  X  48  =  513.66 

The  horse-power  as  above  calculated  is  called  the  indicated 
horse-power  and  is  usually  written  I.  H.  P.  Although  the  above 
calculation  shows  the  amount  of  power  the  engine  develops  it  does 


Fig.  18. 


not  show  the  available  power  since  part  of  the  indicated  horse- 
power is  used  to  run  the  engine  itself,  that  is,  to  overcome  the 
friction  of  the  parts.  To  determine  how  much  power  can  be  used 
to  run  machinery  some  form  of  absorption  dynamometer  or  friction 
brake  is  attached  to  the  engine.  The  power  thus  obtained  is 
called  the  Brake  Horse  Power  or  B.  H.  P.  It  is  more  satisfactory 
for  both  the  owner  and  builder  to  know  the  B.  H.  P.  than  to  know 
the  I.  H.  P. 

The  Prony  Brake,  Fig.  18,  is  one  of  the  simplest  absorption 
dynamometers.  The  two  wooden  blocks  A  and  C  are  held  together 
against  the  rim  of  the  pulley  P  by  bolts.  The  thumb-nuts,  e,  e,  being 


INDICATORS. 


used  to  adjust  the  pressure.  By  means  of  the  bolts  the  arm  L  is 
held  to  the  upper  block.  From  this  arm  is  suspended  the  ball 
weight,  w,  which  by  sliding  along  the  arm  counterbalances  the 
weight  of  the  arm  and  pan  at  the  other  end.  The  pulley  revolves 
at  the  required  speed  in  the  direction  indicated  by  the  arrow. 
The  bolts  are  tightened  until  the  lever  remains  stationary  in  a 
horizontal  position  when  a  known  weight,  W,  is  hung  at  the  end. 

The  amount  of  work  absorbed  by  the  brake  depends  upon  the 
weight  W,  the  length,  R,  and  the  speed.  It  is  independent  of 
the  diameter  of  the  pulley  and  the  pressure  of  the  block  because 
the  moments  of  force  about  the  center  of  the  pulley  are  equal 
when  the  lever  L,  is  horizontal.  Letting/  equal  the  co-efficient 
of  friction,  p  the  pressure  of  the  blocks  and  r  the  radius  of  the 
pulley, 

fpr  =  WR    . 

The  work  done  at  the  face  of  the  pulley  equals  the  force  mul- 
tiplied by  the  distance  or  the  pressure  multiplied  by  the  number 
of  feet  passed  through. 

Let  N  =  the  number  of  revolutions  per  minute.  Then  the 
distance  passed  through  per  minute  equals  2  IT  r  N  and  the  work 
done  equals  2  TT  r  N/J9.  Then  asfp  r  —  W  R,  the  work  done 
at  the  rim  of  the  pulley  equals  the  left  hand  side  of  the  equation 
multiplied  by  2  TT  N,  and  to  keep  both  sides  equal  we  multiply 
W  R  by  2  TT  N.  Hence  the  work  done  is  expressed  by  the  formula 

2  TT  N  W  R  and, 
B  „  p      .   27rNWR 

33,000 
=  .0001904  N  W  R 

A  Prony  brake  with  an  arm  4  feet  long  was  attached  to  the 
pulley  on  the  fly  wheel  of  an  engine.  The  weight  in  the  scale 
pan  was  50  pounds  and  the  speed  of  the  engine  300  revolutions. 
Find  the  brake  horse  power. 

B.  H.  P.  =  .0001904  X  300  X  50  X  4 
=  11.424 

The  rope  brake  shown  in  Fig.  19  is  easily  constructed  of 
material  at  hand  and  being  self-adjusting  needs  no  accurate  fitting. 
For  large  powers  the  number  of  ropes  may  be  increased.  It  is  con- 
sidered a  most  convenient  and  reliable  brake.  In  Fig.  19  the  spring 


30 


INDICATORS. 


balance,  B,  is  shown  in  a  horizontal  position.  This  is  not  at  all 
necessary ;  if  convenient  the  vertical  position  may  be  used.  The 
ropes  are  held  to  the  pulley  or  fly-wheel  face  by  blocks  of  wood,  O. 
The  weights  at  W.may  be  replaced  by  a  spring  balance  if 
desirable. 

To  calculate  the  Brake  Horse  Power,  subtract  the  pull  regis- 
tered by  the  spring  balance,  B,  from  the  load  at  W.  The  lever 
arm  is  the  radius  of  the  pulley  plus  i  the  diameter  of  the  rope. 
The  formula  is, 

B  H  P         27rRN(W  — B) 

33,000 
=         .0001904  R  N  (W  —  B)  * 


Fig.  19. 


Example.  A  rope  brake  is  attached  to  a  gas  engine.  The 
average  reading  of  the  spring  balance  is  8  pounds.  W  =  80 
pounds.  If  the  radius  of  the  brake  wheel  is  28  inches  and  the 
rope  1  inch  in  diameter,  what  is  the  B.  H.  P.  when  the  engine 
makes  350  revolutions  per  minute? 

E  =  28  +  J  =  28i  inches  ==  ^  feet 
B.  II.  P  ==  .0001904  E  N  (V— .B) 

=  .0001904  X  2-^-  X  72  X  350 
=  1*1.4  Ans 
If  both  the  indicated  horse-power  and  the  brake  horse-power 

*  NOTE:  If  B  is  greater  than  W,  the  engine  is  running  in  the  opposite 
direction.  Use  the  formula  B.  H.  P.  =  .0001904  R  N  (B  —  W) . 


INDICATORS.  31 


are  known  the  power  used  in  friction  is  found  by  subtracting  the 
B.  H.  P.  from  the  I.  H.  P. 

The  mechanical  efficiency  of  the  engine  is  the  ratio  of  the 
B.  H.  P.  to  the  I.  H.  P.  or, 

T>       TT        T> 

I.H.P. 

If  an  engine  of  18.2  indicated  horse-power  develops  at  a  trial 
16.02  brake  horse-power,  what  is  its  mechanical  efficiency? 

B.  H.  P. 


E  = 


I.  H.  P. 

16.02 


18.2 
=  88  %  Efficiency. 

Brakes  should  be  well  lubricated.  For  small  powers  the 
heat  generated  by  friction  between  the  ropes  or  blocks  and  the 
rim  of  the  wheel,  will  be  conducted  away  by  radiation  but  for 
large  powers  some  additional  means  is  necessary.  In  case  there 
are  flanges  on  the  wheel,  water  can  be  introduced  into  the  wheel, 
the  flanges  keeping  it  from  flowing  out  and  centrifugal  force  keep- 
ing it  in  contact  with  the  rim.  The  amount  of  water  can  be  regu- 
lated so  that  all  may  be  evaporated,  or  a  scoop  can  be  arranged  to 
carry  off  the  water.  In  all  cases  the  water  should  flow 
continuously. 

To  Find  the  Area  of  Cards.  M.  E.  P.  or  the  mean  effective 
pressure  is  equal  to  the  area  of  the  indicator  diagram  divided  by 
the  length.  The  length  is  easily  found  by  measurement  but  to 
find  the  area  is  more  difficult  since  the  shape  is  irregular.  If  the 
figure  were  regular  its  area  could  be  found  by  geometry  or  by 
simple  formulas. 

The  area  of  the  indicator  card  can  be  found  in  two  ways. 
By  dividing  the  diagram  into  sections  and  by  the  use  of  a  plani- 
meter.  The  former  is  only  an  approximate  method ;  the  area  thus 
found  is  nearly  correct  if  the  number  of  divisions  is  great. 

Tangents  at  each  end,  perpendicular  to  the  atmospheric  line 
are  first  drawn.  The  horizontal  distance  between  these  tangents 
is  then  divided  into  10  or  more  equal  parts.  The  horizontal  length 
of  each  section  is  then  divided  into  two  equal  parts  and  lines  per- 
pendicular to  the  atmospheric  line  drawn  through  these  points  of 


INDICATORS. 


division.  The  sum  of  the  lengths  of  all  these  lines  is  divided  by 
the  number  of  lines  to  get  the  average.  This  average  length  or 
average  ordinate  multiplied  by  the  scale  of  spring  gives  the  mean 
effective  pressure. 

Fig.  20  is  the  card  from  the  crank  end  of  an  engine.     The 


1  3'  5>  7 

Fig.  20. 

line  C  L  is  the  atmospheric  line  and  the  lines  A  D  and  E  F  are 
drawn  perpendicular  to  it  and  tangent  to  the  extreme  ends  of  the 
diagram.  The  line  A  E  is  divided  into  10  equal  parts  and  lines 


Fig.  21. 

are  drawn  through  points  marking  the  centers  of  the  divisions. 
On  each  of  these  lines  the  length  is  marked.  The  sum  of  the 
lengths  is  15.18  and  15.18  divided  by  10  is  1.518.  If  the  scale 
of  spring  is  40  pounds,  1.518  multiplied  by  40  is  the  M.  E.  P.  or 
60.7  ='  M.  E.  P. 


INDICATORS. 


The  Jiorizontal  length  may  be  divided  into  any  number  of 
equal  parts  but  10  or  20  makes  the  computation  easy.  The  oper- 
ation of  finding  the  M.  E.  P.  for  the  head  end  is  exactly  the  same. 
The  average  M.  E.  P.  for  one  revolution  of  the  engine  is  the 
average  of  the  two  mean  effective  pressures. 

In  case  the  diagram  is  very  irregular  it  should  be  divided 
into  20  equal  parts  instead  of  10.  If  there  is  a  loop  in  the  dia- 
gram as  shown  in  Fig.  21  the  area  of  the  loop  must  be  subtracted 


Pig.  22. 

from  the  area  of  the  other  part  as  it  represents  work  done  by  the 
piston  on  the  steam  and  therefore  loss. 

The  lengths  may  be  marked  off  on  a  piece  of  paper  if  a  good 
scale  is  not  at  hand. 

A  more  accurate  result  is  obtained  by  using  an  instrument 
called  the  plani meter.  There  are  several  planimeters  and  aver- 
aging instruments  in  common  use  for  determining  the  mean  effec- 
tive pressure  of  indicator  cards.  The  planimeter  shown  in  Fig. 


34  INDICATORS. 


22  is  one  of  the  most  simple  and  is  called  the  Amsler  Polar  Plain- 
meter  from  its  inventor  Prof.  Amsler.  The  cut  is  about  one-half 
the  size  of  the  instrument.  It  consists  of  two  arms  free  to  move 
about  a  pivot  and  a  roller  graduated  in  inches  and  tenths  of 
inches.  A  vernier  is  placed  with  the  roller  so  the  areas  may  be 
read  in  hundredths  of  a  square  inch.  The  point  A  is  kept  sta- 
tionary and  the  tracer  B  is  moved  once  around  the  outline  of  the 
diagram.  The  area  in  square  inches  of  the  diagram  is  read  from 
the  roller  C  and  the  vernier  E. 

To  Use  the  Planimeter.  The  diagram  should  be  fastened  to 
some  flat  unglazed  surface,  such  as  a  drawing  board,  by  means  of 
thumb  tacks,  springs  or  pins.  The  point  A  is  pressed  into  the 
paper  so  that  it  will  hold  in  place.  The  point  B  is  set  at  any 
point  in  the  outline  of  the  diagram  and  the  roller  set  at  zero. 
Follow  the  outline  of  the  diagram  carefully  in  the  direction  of 
the  hands  of  a  watch  as  indicated  by  the  arrows  in  Fig.  22  until 
the  tracer  has  moved  completely  around  the  diagram.  The  result 
is  then  read  to  hundredths  of  an  inch  from  the  roller.  Suppose 
after  tracing  over  the  outline  we  find  that  the  largest  figure  that 
has  passed  the  zero  of  the  vernier  is  3  ;  the  number  of  graduations 
(tenths)  that  have  passed  the  zero  to  be  5  and  the  number 
(hundredths)  of  the  graduations  in  the  roller  that  exactly  coincides 
with  a  graduation  on  the  vernier  to  be  9.  Then  the  area  is  3.59 
square  inches. 

Often  at  the  start  the  roller  is  not  adjusted  so  that  the  zeros 
coincide  but  the  reading  is  taken  and  subtracted  from  the  final 
reading.  Thus  if  the  first  reading  is  4.63  and  the  second  7.31  the 
area  is  7.31  • —  4.63  =  2.68  square  inches.  In  case  the  second 
reading  is  less  than  the  first,  add  10  to  the  second  reading  then 
subtract. 

This  instrument  is  very  valuable  to  an  engineer  who  takes 
indicator  cards.  The  results  obtained  are  very  accurate,  the  error 
being  small.  Ten  or  twelve  diagrams  can  be  measured  by  this 
instrument  in  the  same  time  that  is  necessary  to  measure  a  single 
card  by  the  method  of  ordinates. 

It  is  well  to  run  over  the  area  three  or  four  times  and  take 
an  average  as  the  tracing  of  the  diagram  cannot  be  absolutely  cor 
rect  at  any  time. 


INDICATORS.  35 


THERflAL  EFFICIENCY. 

The  thermal  efficiency  of  the  steam  engine  is  found  in  the 
same  manner  as  that  of  any  other  heat  engine.     The  efficiency 
depends  upon  the  limits  of  temperature  and  not  upon  the  nature 
of  the  working  medium. 
Let  Tj  =  absolute  temperature  of  the  heat  received  by  the  engine. 

T2  =  absolute  temperature  of  the  heat  rejected  by  the  engine. 

E     =  efficiency  of  engine. 
Then, 

E  =    TI  "  '  T2 


or,  the  efficiency  equals  the  temperature  of  the  heat  rejected,  sub- 
tracted from  the  temperature  of  the  heat  received  and  the  result 
divided  by  the  temperature  of  the  heat  received. 

Suppose  an  engine  is  supplied  with  steam  at  120  pounds 
absolute  pressure  and  the  exhaust  is  atmospheric  pressure.  What 
is  the  efficiency? 

The  absolute  temperature  corresponding  to  120  pounds  abso- 
lute pressure  is  341.05°  -|-  461°  and  the  temperature  of  atmos- 
pheric pressure  is  212°  -f-  461°. 

Then, 

802.05  —  673 
E  =  -        _  =  .16  or  16  per  cent. 

802.05 

If  the  engine  had  been  of  the  condensing  type  and  the 
exhaust  pressure  one  pound  above  the  vacuum,  the  efficiency 
would  be  as  follows  : 

The  temperature  of  one  pound  absolute  pressure  is  101.99° 
+  461°. 

-,         802.05  —  562.99 


-8005- 

In  actual  engines  this  efficiency  cannot  be  obtained  because 
the  difference  between  the  amount  of  heat  received  and  that 
rejected  is  not  all  converted  into  work.  Part  of  it  is  lost  by 
radiation,  conduction,  leakage,  etc.  Also  cylinder  condensation 
reduces  the  efficiency. 

The  Theoretical  Indicator  Diagram.  An  indicator  diagram 
is  the  result  of  two  movements  ;  a  horizontal  movement  of  the 


3(5  INDICATORS. 


paper  and  a  vertical  movement  of  the  pencil.  The  horizontal 
movement  exactly  corresponds  to  the  movement  of  the  piston  of 
the  engine  and  the  vertical  movement  exactly  corresponds  to  the 
pressure  of  steam  in  the  cylinder. 

The  shape  of  the  indicator  card  depends  upon  the  manner  in 
which  steam  is  admitted  to  and  released  from  the  cylinder.  Dif- 
ferent engines  give  different  shaped  indicator  cards  and  the  cards 
taken  from  an  engine  vary  with  the  conditions.  Figs.  1  and  2 
show  theoretical  indicator  cards  from  a  non-condensing  engine 
without  clearance ;  the  former  being  for  the  case  that  has  admis- 
sion during  the  whole  stroke.  The  diagram  of  Fig.  2  shows  the 
cut  off  at  4-  stroke.  All  practical  engines  have  clearance  and 
slight  compression ;  so  the  theoretical  diagram  assumes  the  shape 

shown  in  Fig.  23.  In  this  card 
the  admission  line  H  A  is  verti- 
cal, the  steam  line  A  C  is  hori- 
zontal, the  expansion  line  C  D 
an  hyperbolic  curve,  the  exhaust 
line  D  B  vertical,  the  back  pres- 
sure line  B  F  horizontal  and  the 
compression  curve  an  hyperbola. 
The  actual  shape  is  somewhat 
different  from  the  theoretical 

mainly  because  the  valves  do  not  open  and  close  quickly,  the  ports 
offer  some  resistance  to  the  passage  of  the  steam  and  the  back 
pressure  is  neither  atmospheric  in  the  non-condensing  engine  nor 
absolute  vacuum  in  the  condensing  engine. 

The  diagram  shown  in  Fig.  24  is  a  practical  diagram  and 
like  those  taken  from  engines. 

The  atmospheric  line  L  M  is  the  line  drawn  by  the  pencil  of 
the  indicator  when  the  connection  to  the  engine  is  closed  and  both 
sides  of  the  piston  of  the  indicator  are  open  to  the  atmosphere. 
It  is  the  zero  of  the  steam  gage. 

The  admission  line  H  A  shows  the  rise  of  pressure  due  to  the 
admission  of  steam  to  the  cylinder.  If  the  steam  is  admitted 
qnickty  when  the  engine  is  nearly  on  dead  center  this  line  will  be 
very  nearly  vertical, 

The  steam  line  A  C  is  drawn  while  the  valve  admits  steam 


INDICATORS. 


37 


to  the  cylinder.  This  line  is  horizontal  if  there  is  no  wire- 
drawing. 

The  point  of  cut  off  C,  indicates  the  point  at  which  the 
admission  of  steam  is  stopped  by  the  closing  of  the  valve.  This 
point  is  rounding  since  the  valve  closes  slowly.  Sometimes  it  is 
difficult  to  determine  the  exact  point  where  cut  off  takes  place ;  it 
is  usually  where  the  curve  changes  from  concave  to  convex. 

The  expansion  curve  C  D  shows  the  fall  in  pressure  as  the 
bteam  expands  while  the  piston  moves  toward  the  end  of  the  stroke. 

The   point  of  release  D  shows  the  point  at  which  the  exhaust 


N 


M 


rig.  24. 

valve  opens.  The  rounding  is  due  to  the  slow  action  of  the  valve 
when  opening.  Because  of  this  slow  action  of  the  valve,  release 
begins  a  little  before  the  end  of  the  forward  stroke. 

The  exhaust  line  D  E  F  represents  the  loss  in  pressure  which 
occurs  while  the  valve  opens  to  exhaust  at  and  near  the  end  of  the 
stroke. 

The  back  pressure  line  F  G  shows  the  back  pressure  against 
which  the  piston  acts  during  the  return  stroke.  For  a  condens- 
ing engine  this  line  is  below  the  atmospheric  line  L  M,  the  dis- 
tance below  being  dependent  upon  the  state  of  the  vacuum  in  the 
condenser.  For  cards  taken  from  a  non-condensing  engine  the 
back  pressure  line  is  a  little  above  the  atmospheric  line. 

The  point  of  exhaust  closure  G  is  the  point  where  the  valve 


38 


INDICATORS. 


closes  to  exhaust.  The  exact  point  is  not  clearly  defined  as  the 
curve  shows  a  change  of  pressure  due  to  the  gradual  closing  of  the 
valve. 

The  compression  curve  G  H  shows  the  rise  of  pressure  due 
to  the  compression  of  the  steam  remaining  in  the  cylinder  after 
the  valve  lias  closed  to  exhaust. 

The  zero  line  of  pressure  or  line  of  absolute  vacuum  O  X  is 
drawn  below  and  parallel  to  the  atmospheric  line.  The  distance 
between  the  lines  O  X  and  L  M  represents  14. T  pounds  pressure. 

The  clearance  line  O  Y  is  drawn  perpendicular  to  the  line  of 
absolute  vacuum  and  at  a  distance  from  the  end  of  the  diagram 


, ,--}--  | 4—*-- 


PR  X 

Fig.  2o. 

equal  to  the  same  per  cent,  of  the  length  of  the  diagram  as  the 
clearance  volume  is  of  the  piston  displacement,  or 
L  N  clearance  volume 

L  M  volume  of  cylinder 

It  is  readily  seen  that  the  area  of  an  actual  indicator  diagram 
is  less  than  that  of  a  theoretical  card.  This  is  because  of  the 
round  corners  at  cut  off  and  exhaust,  the  back  pressure  and  the 
compression.  Sometimes  it  is  useful,  especially  in  designing 
engines,  to  draw  the  theoretical  indicator  card. 


INDICATORS.  39 


To  Draw  the  Theoretical  Card.  To  draw  an  ideal  diagram 
draw  P  X  equal  to  the  length  of  stroke  and  O  P  equal  to  the 
clearance  shown  in  Fig.  25.  Draw  O  Y  and  P  A  perpendicular 
to  O  X  and  draw  Y  S  parallel  to  O  X  and  at  a  height  correspond- 
ing to  the  boiler  pressure. 

The  line  of  initial  pressure  A  C  is  then  drawn  parallel  to  Y  S 
and  is  usually  taken  as  from  90  to  95  per  cent,  of  the  boiler  pres- 
sure if  there  is  no  special  cause  for  loss.  Then  take  A  C  as  the 

P  X 

portion  of  the  stroke  at  which  steam  is  admitted  so  that = 

A  O 

the  ratio  of  expansion.  The  expansion  line  is  considered  a  hyper- 
bolic curve  with  O  Y  and  O  X  as  asymptotes.  To  draw  the 
hyperbolic  curve.  First  draw  the  line  A  C  B  parallel  to  the 
atmospheric  line  and  F  D  B  and  R  C  perpendicular  to  it.  Then 
make  points  1,  2,  3,  4,  etc.,  on  C  B  and  connect  them  with  the 
point  O.  At  the  points  1',  2',  3',  4',  etc.,  where  these  lines  inter- 
sect the  line  R  C  draw  parallels  to  C  B  until  they  meet  perpen- 
diculars from  1  *  2,  3,  4.  The  point  of  intersection  of  these  lines 
are  points  in  the  hyperbolic  curve  C  D,  as  shown  in  Fig.  25. 
Any  number  of  points  may  be  used ;  but  there  must  be  enough  to 
determine  the  curve. 

The  area  A  C  D  F  N  H  is  the  theoretical  card,  with  a  given 
boiler  pressure,  ratio  of  expansion  and  an  assumed  back  pressure. 
The  actual  card  for  the  same  data  would  be  more  nearly  like  the 
shaded  area  which  lies  mostly  within  the  outline  of  the  theoretical 
card.  In  designing  engines  it  is  well  to  know  the  ratio  of  the 
actual  to  the  ideal  card  for  all  types  of  engines.  .  , 

This  ratio  varies  with  the  speed,  type  of  engine,  and  kind  of 
valves  and  has  the  following  values. 

For  ordinary  plain  slide  valve  engines,  .8    to  .9 

For  small  engines,  about,  .85 

For  large  engines,  about,  .90 

For  engines  with  high  speed,  about,  .75 

For  compound  engines,  .75  to  .80 

For  compound  engines,  high  speed,  .65  to  .75 

For  triple  expansion  engines,  .60  to  .70 

For  triple  expansion  engines,  high  speed,  .50  to  .60 


40 


INDICATORS. 


CARDS  FOR  COMPOUND  ENGINES. 

In  Fig.  26  the  ideal  cards  of  a  tandem  compound  engine  of 
the  Woolf  type  are  shown.  The  diagram  ACDEFGH  is 
from  the  high  pressure  cylinder  and  the  diagram  L  M  N  P  Q  R 
from  the  low  pressure.  S  T  is  the  atmospheric  line. 

The  line  H  A  is  the  admission  line,  A  C  the  steam  line,  and 
C  D  the  expansion  line.  These  lines  are  similar  to  those  of  a 
single  cylinder  engine  diagram.  At  D,  the  point  of  release,  the 
pressure  drops  slightly  as  steam  is  admitted  to  the  low  pressure 


Fig.  26. 

cylinder.  The  back  pressure  line  E  F,  of  the  high  pressure  cylinder 
is  parallel  to  the  steam  line  L  M,  of  the  low  pressure  cylinder. 
They  would  coincide  if  there  were  no  resistance  to  the  flow  of 
steam  and  no  heat  loss.  The  flow  of  steam  from  the  high  to  the 
low  pressure  cylinder  is  cut  off  at  F  and  the  steam  remaining  in 
the  high  pressure  cylinder  is  compressed  in  the  cylinder  and  in  the 
pipes.  The  exhaust  closes  at  G  and  steam  is  compressed  in  the 
small  cylinder  from  G  to  H. 


INDICATORS. 


41 


Cut  off  occurs  in  the  low  pressure  cylinder  at  M,  and  the 
steam  in  the  cylinder  expands,  the  curve  M  N  being  an  equilateral 
hyperbola.  Release  takes  place  at  N  and  the  pressure  falls  to 
that  of  the  condenser.  The  back  pressure  line  P  Q  and  the  com- 
pression curve  Q  K  are  like  those  of  a  single  engine. 

The  diagrams  shown  in  Fig.  27  are  from  a  Woolf  engine. 
The  lines  are  similar  to  those  of  the  theoretical  diagram  shown  in 
Fig.  26. 

With  compound  engines  of  the  Woolf  type  the  steam  passes 


Fig.  27. 

directly  from  the  high  pressure  cylinder  to  the  low  pressure.  In 
the  cross  compound,  the  cranks  being  at  90°,  the  piston  in  one 
cylinder  is  at  the  end  of  the  stroke  when  the  other  is  at  the  middle 
of  the  stroke.  Therefore,  a  receiver  must  be  used.  The  ideal 
cards  from  a  cross  compound  engine  are  shown  in  Fig.  28.  The 
dimensions  are  as  follows  : 

Volume  of  high  pressure  cylinder 

Volume  of  receiver 

Volume  of  low  pressure  cylinder 

Cut  off  of  high  pressure  cylinder 

Cut  off  of  low  pivssure  cylinder 

Initial  pressure  (absolute) 

Back  pressure  (absolute) 


=  5  cu.  ft. 

=  5  cu.  ft. 

=  15  cu.  ft. 

=  |  stroke. 

—  ^-  stroke. 
=120  Ibs. 

=»  3  Ibs. 


42 


INDICATORS. 


Clearance  and  compression  are  not  considered.  The  stroke 
of  the  high  pressure  cylinder  begins  at  A.  Steam  is  cut  off  at  ^ 
stroke  at  C. 

Since  steam  is  cut  off  at  ^  stroke,  the  ratio  of  expansion  in 
the  high  pressure  cylinder  is  4.  Then  the  volume  of  steam  at  D 
is  4  times  that  at  C  and  since  p  v  =  p^  vl,  the  pressure  at  I)  is 


OQ    vx     1 

___  —  80  pounds.      Steam  is  released  at  D  and  passes  to  the 


receiver.  The  pressure  at  release,  L,  of  the  low  pressure  cylinder 
is  found  from  the  equation  p  v  =  pl  v1.  The  volume  of  steam 
in  the  high  pressure  cylinder  at  cut  off,  is  |  cubic  feet  and  the 

C,  -  .A 


volume  at  L  is  15  cubic  feet;  then  the  total  ratio  of  expansion  is 

120  X  1 
15  -r-  | •  =  12.     Then  the  pressure  atL  is L  =  10  pounds. 

Since  the  low  pressure  cylinder  cuts  off  at  J  the  stroke  the 
volume  at  G  is  J  that  at  L. 

Then  as  p  v  —  pl  v^  p  X  \  =  10  X   1,  or  p  =  20  pounds 

Hence  the  pressure  at  G  is  20  pounds. 

We  can  now  find  the  pressure  at  E.  The  pressure  in  the 
receiver  when  the  low  pressure  cylinder  cut  off,  was  20  pounds 
because  the  low  pressure  cylinder  was  in  communication  with  it- 


INDICATORS.  43 


Then  as  the  piston  in  the  small  cylinder  is  at  the  end  of  the  stroke 
when  the  steam  is  cut  off  in  the  large  cylinder,  steam  at  30 
pounds  pressure  rushes  from  the  high  pressure  cylinder  into  the 
receiver  and  mixes  with  steam  at  20  pounds  pressure.  When 
steam  is  cut  off  in  the  low  pressure  cylinder  the  high  pressure 
cylinder  and  the  receiver  are  in  communication  with  each  other  ; 
the  total  volume  being  the  volume  of  the  high  pressure  cylinder 
plus  that  of  the  receiver  or  5  -f-  5  —  10  cubic  feet.  What  we 
wish  to  find  is  the  pressure  of  the  steam  of  this  volume.  The 
formula  is, 

P  V  =  p  v  +  pl  v^ 

Since  there  are  10  cubic  feet  in  the  receiver  and  high  pressure 
cylinder,  the  value  of  V  is  10.  The  volume  of  steam  in  the 
high  pressure  cylinder  is  5  cubic  feet  and  its  pressure  is  30  pounds. 
The  volume  of  steam  in  the  receiver  is  5  cubic  feet  and  its  pressure 
is  20  pounds,  then, 

10  P  =  5  X  30  +  5  X  20 


P  =  _  =  25  pounds. 

Then  the  pressure  at  E  =  25  pounds. 

While  the  piston  of  the  high  pressure  cylinder  is  on  the  return 
stroke,  steam  in  that  cylinder  is  compressed  from  E  to  F  ;  the 
volume  being  10  cubic  feet.  When  the  piston  has  completed  ^ 
the  return  stroke,  the  volume  at  F  is  equal  to  J  the  volume  of  the 
high  pressure  plus  the  volume  of  the  receiver  or  f  -\-  5  =  7-J  cubic 
feet. 

Then  with  the  formula  p  v—p^  v-^  we  obtain, 

25  X  10        OQ  oo 
p  -         7JL         =  33.33 

or  the  pressure  at  F  is  33.33  pounds. 

Then  as  the-'cranks  are  90°  apart,  and  the  high  pressure  piston 
is  at  the  middle  of  the  stroke,  the  low  pressure  piston  must  be  at 
the  beginning  of  its  stroke  ;  or  the  pressure  in  the  low  cylinder  is 
the  same  as  that  in  the  high  and  in  the  receiver,  or  33.33  pounds* 
Since  the  stroke  is  beginning  in  the  low  pressure  cylinder,  steam 
is  being  admitted  to  it  from  the  receiver  and  consequently  the 
pressure  in  the  large  cylinder  falls  as  the  volume  increases,  which 


INDICATORS. 


is  shown  by  the  line  F  G.  Cut-off  occurs  in  the  low  pressure 
cylinder  at  G,  and  the  steam  expands  until  the  piston  reaches  L  ; 
the  curve  being  an  equilateral  hyperbola.  At  L  the  release  occurs 
and  the  pressure  drops  to  that  in  the  condenser  ;  in  this  case  about 
3  pounds.  The  back  pressure  line  is  of  course  the  pressure  in  the 
condenser. 

The  cards  of  Fig.  29  are  from  a  cross  compound  engine.     The 
rise  of  pressure  in  the  middle  of  the  back  pressure  line  of  the 


Fig.  29 

diagram  from  the  small  cylinder  is  due  to  the  fluctuation  of  pres- 
sure in  the  receiver. 

COflBINED  DIAGRAHS. 

The  indicator  cards  of  multi-cylinder  engines  may  be  com- 
bined so  that  the  pressures  and  volumes  are  shown  in  their  proper 
relations.  To  do  this,  the  cards  are  reduced  to  the  same  scale  of 
pressure  and  the  same  scale  of  volume.  To  make  the  combined 
diagram  of  convenient  size,  the  length  of  the  low-pressure  card  is 


INDICATORS. 


45 


left  as  it  is  and  the  length  of  the  high-pressure  diagram  is  shortened 
in  the  ratio  of  the  cylinder  volumes. 

Perhaps  the  best  way  to  show  the  method  is  by  an  illustra- 
tion. The  cards  shown  in  Fig.  30  were  taken  from  a  compound 
condensing  engine. 

Ratio  of  cylinder  volumes    =1:3 
Initial  pressure  =  138  pounds 

Spring,  high-pressure  card  —    60  pounds 
Spring,  low-pressure  card  =    30  pounds 


Fig.  30. 

First  draw  the  line  of  zero  pressure  A  B  and  the  line  of  zero 
volume  E  F  (see  Fig.  31).  The  line  O  P  is  drawn  parallel  to  E  F 
and  at  a  distance  proportional  to  the  clearance  of  the  low-pressure 
cylinder.  Similarly  the  distance  between  M  N  and  E  F  represents 
the  clearance  of  the  high-pressure  cylinder.  Now  draw  the  atmos- 
pheric line  CD.  In  this  case  it  will  be  .49  inch  above  A  B 

1  A  7 

because  a  30-pound  spring  is  used,  and  . — 1-  =.49.    Reproduce  the 

oO 

low-pressure  card  without  change,  as  shown. 

Divide  the  high-pressure  card  with  10  (or  more)  ordinates, 
and  reproduce  it  with  volumes  and  pressures  of  the  same  scale  as 
the  low-pressure  card.  Since  a  60-pound  spring  was  used,  each 

f\C\ 

ordinate  will  be  twice  as  long  (because  _  =  2).      The  distance' 


46 


INDICATORS. 


between  the  ordinates  will  be  |-  as  great  because  the  high-pressure 
cylinder  is  1  the  volume  of  the  low.  Draw  the  ordinates  as  shown. 
The  distance  between  ordinates  L  and  M  will  be  ^  of  the  length 
of  the  high-pressure  card.  The  points  c  and  e  are  on  the  fifth 
ordinate,  and  are  twice  as  far  above  the  atmospheric  line  as  they 
are  in  the  original  high-pressure  card. 

After  locating  all  the  points  draw  the  curve  through  them. 
Now  draw  the  theoretical  expansion  curve  R  S  through  the  point 
of  cut-off  of  the  high-pressure  cylinder  by  the  method  explained 


Fig.  31. 

on  page  39.  The  difference  between  the  area  included  between 
the  theoretical  curve  and  the  lines  of  no  pressure  and  no  volume, 
and  the  sum  of  the  actual  areas,  represents  approximately  the 


This  method  is  not  strictly  accurate,  because  all  the  steam 
used  in  the  high-pressure  cylinder  does  not  pass  to  the  low-pressure 
cylinder :  a  small  portion  is  left  for  compression.  By  thus  com- 
bining the  cards  the  action  of  the  valves  may  be  discussed,  provided 
such  data  as  size,  type  and  speed  are  known. 

The  combined  diagram  of  a  multi-expansion  engine  drawn 


INDICATORS'. 


47 


according  to  the  above  method  is  shown  in  Fig.  32.  The  volumes 
and  pressures  are  first  reduced  to  the  scale  of  the  low-pressure 
card.  The  cards  are  then  redrawn  at  the  proper  distances  from 
the  lines  of  no  pressure  and  no  volume ;  the  clearance  in  each 
cylinder  being  considered. 


Fig.  32 
HORSE=POWER  OF  COMPOUND  ENGINES. 

The  I.  H.  P.  of  multi-cylinder  engines  may  be  found  by  add- 
ing the  I.  H.  P.  of  the  several  cylinders.  Another  method  is  to 
reduce  all  the  pressu^s  to  the  area  of  the  low-pressure  cylinder. 
This  is  done  by  dividing  the  M.  E.  P.  of  each  cylinder  by  the 
inverse  ratio  of  ics  volume  to  that  of  the  low-pressure  cylinder. 

Suppose  the  M  E.  P.  of  the  high-pressure  cylinder  of  a  com- 
pound engine  is  78  pounds  as  found  from  the  indicator  card.  It 
the  volume  of  the  high-pressure  cylinder  is  1  that  of  the  low,  the 

78 
M.  E.  P.  of  the  high  referred  to  the  low  would  be  —  =  26  pounds. 

o 

Then  if  the  card  from  trie  large  cylinder  shows  a  M.  E.  P.  of  30 
pounds,  the  total  M.  E.  P.  is  30  -f-  26  —  56  pounds,  and  the 
I.  H.  P.  is  found  by  inserting  56  as  the  value  of  P  and  using  the 
area  of  the  low  pressure  as  A,  in  the  formula  for  L  H  P. 


48  INDICATORS. 


INDICATOR  CARDS. 

The  indicator  reveals  defects  in  the  steam  distribution.  That 
is,  if  the  valve  or  valves  are  set  so  that  the  events  of  the  stroke 
are  too  early  or  too  late  or  if  more  work  is  done  at  one  end  of  the 
cylinder  than  at  the  other,  the  engineer  finds  it  out  by  examining 
the  indicator  card. 

Sometimes  an  engine  appears  to  run  well  and  the  owner  is 
perfectly  satisfied  with  it ;  but  from  the  indicator  diagrams  it  is 
found  that  considerable  saving  might  be  effected  by  correcting  the 
defects  in  the  valve  setting. 

On  looking  at  a  diagram  one  might  say  it  was  a  faulty  card 


B 


Fig.  33. 

and  yet  for  that  type,  size  and  speed  it  is  perhaps  the  best  that 
could  be  obtained  from  the  engine.  The  same  form  of  diagram  is 
not  possible  from  different  types  of  engines.  The  diagram  from  a 
Corliss  engine,  having  four  valves,  is  different  from  that  of  the 
plain  slide  valve  engine  ;  also  the  diagrams  from  high  speed  engines 
differ  from  those  of  low  speed. 

The  most  common  faults  in  the  distribution  of  steam  in  the 
cylinder  can  be  divided  into  four  classes. 

a  =  Admission  too  early  or  too  late. 
/>  —  Cut-off  too-early  or  too  late. 
c  =  Release  too  early  or  too  late. 
d  —  Compression  too  early  or  too  late. 
In  the  following  figures, 

A  is  the  point  of  admission, 
C  is  the  point  of  cut  off, 
B  is  the  point  of  release, 
G  is  the  point  of  compression, 


INDICATORS. 


49 


The  diagram  shown  in  Fig.  33  shows  too  early  admission. 

The  admission  line  curves  backward  instead  of  being  straight  and 
perpendicular  to  the  atmospheric  line  as  it  is  in  Fig.  24.  The 
diagram  also  shows  cut  off,  release  and  compression  early.  When 
the  valve  is  of  the  plain  slide  valve  type  all  the  events  are  likely 
to  be  too  early  if  one  of  them  is.  "The  reason  for  the  event  being 


Fig.  34. 

too  early  is   that  the   eccentric   has   too   much   angular   advance, 
Hence  the  remedy  is  to  decrease  the  angular  advance. 

Fig.  34  shows  a  diagram  having  the  opposite  defects  to  those 
of  Fig.  33.  The  events  are  too  late.  The  admission  line  curves 
forward  and  the  line  shows  that  admission  does  not  take  place 
until  after  the  stroke  is  well  begun.  Release  occurs  at  the  end  of 


B 


Fig.  35. 

the  strike.  In  this  case  the  angular  advance  of  the  eccentric 
should  be  increased  until  the  admission  line  is  perpendicular  to  the 
atmospheric  line. 

Fig.  35  shows  a  card  having  too  much  back  pressure.     This 
may  be  due  to  a  small  exhaust  port  or  pipes,  or  the  passage  of  steam 


50 


INDICATORS. 


through  coils  of  pipe  for  heating.  The  card  shows  the  other  events 
to  be  good.  A  diagram  having  too  late  cut  off  is  shown  in  Fig. 
36.  The  pressure  at  release  is  high.  When  the  engine  is  running 
under  this  condition  much  of  the  benefit  from  expansion  is  lost. 


B 


Fig.  30. 

A  diagram  from  a  condensing  engine  is  shown  in  Fig.  37. 
These  oscillations  are  caused  by  the  vibration  of  the  indicator 
piston  and  spring.  To  avoid  these  vibrations  never  use  a  very 
light  spring  for  high  speed. 

The  diagram  of  Fig.  21  shows  too  early  cut  off.  In  this  case 
the  cut  off  is  so  early  that  the  expansion  line  extends  below  the 


atmospheric  line  making  a  loop.     The  area  of  the  loop  must  be 
subtracted  from  the  area  of  the  card  as  explained  on  page  33. 

Fig.  38  shows  a  pair   of  diagrams   from   a   plain   slide   valve 
engine.     The  admission  lines  are  good.      The  sloping  steam  lines 


INDICATORS. 


51 


show  wire-drawing  due  to  the  slow  action  of  the  valve  or  too 
small  ports  or  pipes.  This  wire-drawing  decreases  the  area  of  the 
card  which  indicates  loss.  The  greatest  fault  is  the  inequality  of 
area  of  the  diagram  for  the  ends.  The  late  cut  off  and  conse- 
quent late  compression  of  one  end  causes  more  area  than  the  too 
early  cut  off  and  too  early  compression  of  the  other  end.  These 


Fig.  38. 

cards  can  be  improved  by  adjusting  the   angular  advance  of  the 
eccentric  and  the  length  of  the  valve  rod. 

The  diagram  of  Fig.  39  indicates  too  early  compression.  The 
compression  curve  extends  above  the  initial  pressure  line.  The 
area  of  the  loop  must  be  subtracted  from  the  card  area,  when  com- 


B 


Fig.  39. 

puting  the  I.  H.  P.  If  the  cut  off  is  kept  the  same  and  the  com- 
pression made  what  it  should  be,  the  gain  in  area  would  be  the 
area  included  -between  the  full  line  and  the  dotted  line  plus  the 
urea,  of  the  loop.  The  remedy  for  this  case  is  to  decrease 
the  inside  lap. 


52  INDICATORS. 


The  amount  of  compression  varies  with  the  speed  and  type  of 
engine.  Slow  speed  engines  require  less  compression  or  cushion- 
ing than  high  speed.  The  exhaust  steam  should  never  be  com- 
pressed higher  than  the  boiler  pressure. 

For  high  speed  engines  the  compression  should  extend  to 
about  .9  the  initial  pressure.  For  medium  speeds  about  ./>  and  for 
low  speed  .2  to  .4. 

In  the  case  of  a  slide  valve  engine  it  is  not  always  possible  to  set 
the  valve  so  that  the  card  may  have  all  the  events  as  they  should 
be.  Sometimes  the'  laps  of  the  valves  should  be  altered.  For  too 
much  compression  decreases  the  inside  lap.  For  too  early  cut  off 
decrease  the  outside  lap. 

If  the  valve  travel  is  increased,  compression  is  retarded,  that 
is,  decreased ;  release  occurs  sooner. 

STEAM  CONSUMPTION. 

The  amount  of  steam  used  by  an  engine  is  called  its  Steam 
consumption  and  for  comparison,  it  is  customary  to  state  the 
amount  of  steam  consumed  per  indicated  horse-power  per  hour. 
By  means  of  the  indicator  diagram  the  steam  consumption  can  be 
computed  approximately. 

To  find  the  Steam  Consumption  from  the  Diagram.     The 

diagram  shown  in  Fig.  40  is  from  a  20  X  36  engine,  running  at  a 
speed  of  80  revolutions  per  minute.     A  40-pound  spring  was  used. 

By  measuring  the  card,  we  find  the  mean  ordinate  to  be  .91 
inch  and  the  M.  E.  P.  =  .91  X  40  ----  36.4  pounds. 

I.  H.  P.  =  Engine  Constant  X  M.  E.  P.  X  piston  speed. 
;=  .00952  X  36.4  X  480. 
=  166.33. 

In  Fig.  40  L  M  is  the  atmospheric  line  and  O  R  the  line 
of  zero  pressure  drawn  so  that  O  L  -=  14.7  pounds.  O  N  is  tin1 
clearance  volume  —  8  per-  cent  of  the  piston  displacement.  The 
line  P  Q  is  drawn  from  O  li  to  some  point  011  the  compression 


INDICATORS. 


53 


curve.     From  D,  a  point  on  the  expansion  curve  before  release, 
the  line  D  F  is  drawn  perpendicular  to  O  R. 

Then  from  the  diagram,  * 

O  R  —  3.24    inches. 
O  F  =  3.00    inches. 
O  P  =     .345  inch. 
D  F  =     .795  inch. 
P  Q  —     .795  inch. 

The  length  of  stroke  is  36  inches  or  3  feet,  and  the  length  of 
the  diagram  3  inches.  Then  1  inch  of  the  length  of  the  card  cor- 
responds to  1  foot  of  the  stroke.  The  scale  of  spring  used  is  40. 
Therefore  we  can  easily  reduce  the  above  dimensions  to  pounds 
pressure  and  to  feet. 

O  R  =     3.24    feet. 
O  F  =     3.00    feet. 
OP—      .345  feet. 
D  F  =  31.80    pounds. 
P  Q  =  31.80    pounds. 
The  area  of  the  piston  (head  end)  is, 

Z*L  =  8'1416><  4<>Q  =  314.16  sq.  in.  =  2.18166  so,  ft. 


Fig.  40. 

We  can  now  find  the   volume  of  steam  at  any  point  of  the 

irrroke. 

When  the  piston  is  at  1),  the  volume  is, 

2.18166  X  3  =  6.54498  cubic  feet. 


54  INDICATORS. 


When  the  piston  is  at  Q,  the  volume  is, 

2.18166  x  -345  —  .75267  cubic  foot. 

From  the  steam  tables  we  can  find  the  weight  of  a  cubic  foot 
of  steam  at  a  given  pressure. 

The  weight  of  1  cubic  foot  at  31.8  pounds  absolute  pressure 
is  .07773  pound.  Then  the  weight  of  steam  present  when  the 
piston  is  at  D  is, 

G.54498  X  .07773  ==  .50887  pound. 
The  weight  of  steam  present  when  the  piston  is  at  Q  is 

.75267  X  .07773  ==  .05852  pound. 

The  weight  of  steam  in  the  cylinder  is  .50887  pound  and  the 
weight  of  steam  kept  for  compression  is  .05852  pound.  The 
weight  exhausted  per  stroke  is, 

.50887  —  .05852  =  .45035  pound. 
The  amount  used  per  I.  H.  P.  per  hour  is, 

.45035  X  2  X  80  X  60    ±  ^ 

166.33 

This  may  be  stated  in  words  as  follows : 

Measure  the  pressure  from  the  vacuum  line  to  some  point  in 
the  expansion  curve  before  release  and  from  the  steam  tables  find 
the  weight  of  a  cubic  foot  at  that  pressure.  Multiply  the  volume 
(in  cubic  feet)  of  the  cylinder,  including  clearance  to  that  point, 
by  the  weight  per  cubic  foot.  The  result  is  the  weight  of  steam 
in  the  cylinder.  As  this  weight  includes  the  steam  used  for  com- 
pression it  must  be  corrected  to  obtain  the  weight  used  per  stroke. 
Take  some  point  on  the  compression  curve  and  measure  its 
absolute  pressure.  Then  compute  the  weight  of  steam  to  this 
point.  Subtract  this  weight  from  the  weight  of  steam  to  the 
point  in  the  expansion  curve  and  the  result  is  the  weight  of  steam 
used  per  stroke. 

Multiply  the  weight  of  steam  used  per  stroke  by  the  number 
of  strokes  and  divide  by  the  indicated  horse-power  as  found  from 
the  card.  The  final  result  is  the  number  of  pounds  of  steam  con- 
sumed per  horse-power  per  hour. 

We  may  also  calculate  the  steam  consumption  by  taking  the 
point  of  the  expansion  curve  near  the  cut-off. 

O  B  =  1.23  inches  =  1.23  feet  of  the  stroke. 
B  C  ='1.8    inches  =  72  pounds. 


INDICATORS.  55 


The  weight  of  1  cubic  foot  of  steam  at  72  pounds  absolute 
pressure  is  .1671  pounds.     Then  the  volume  of  steam  at  C  is, 

2.18166  X  1.23  =  2.68341  cubic  feet. 
The  weight  at  C  is, 

2.68344  X  .1671  =  .44840  pound. 

The  weight  kept  for  compression  is  the  same  as  previously 
found,  L  e.,  .05852  pound. 

Then  the  weight  of  steam  used  per  stroke  is, 

.44840  —  .05852  =  .38988  pound. 
The  steam  consumption  per  I.  II.  P.  per  hour  is, 

.38988  X  2  X  80  X  60 


166.33 


=  22.50  pounds. 


If  the  valve  doesn't  leak,  the  amount  of  sfeam  just  after  cut 
off  should  equal  the  amount  just  before  release,  but  our  calculation 
shows  tlui4;  there  is  .45035  --  .38988  =  .06047  pound  more  at 
release  than  at  cut  off.  This  shows  that  at  entrance  .06047  pound 
was  condensed  before  the  piston  reached  C  and  was  re-evaporated 
before  release.  This  calculation  gives  an  idea  of  the  amount  of 
cylinder  condensation. 

If  there  is  considerable  compression  as  hi  I^ig.  40  the  above 
method  may  be  simplified  by  taking  the  two  points  D  and  Q 
at  the  same  height  above  the  vacuum  line.  The  pressures  will 
then  be  the  same. 

Let  W  =  weight  of  steam  used  per  I.  II.  P.  per  hour. 

w  —  weight  of  one  cubic  foot  of  steam  at  the  absolute 

pressure  D  F. 

L  =  length  of  the  diagram,  N  R. 
/  —  distance  between  Q  and  D. 
P  —  mean  effective  pressure. 

Then, 

w  _      13,750  XwXl 
P  L 

Example.  A  card  was  taken  from  a  1 2  X  14  engine.  Length 
of  card  L  —  3.5  inches,  I  —  2.875  inches.  Absolute  pressure  at 


56  INDICATORS. 


D  =  30  pounds.     M.  E.  P.  =  30.75  pounds.     What  is  the  steam 
consumption  per  I.  H.  P.  per  hour  ? 

13,750  w  X  I 


•Mr   


P  X  L 
13.750  X  .0736  x  2.875 


30.75  X  3.5 
—    27.03  pounds. 

This  method  gives  only  an  approximate  steam  consunr  don 
On  account  of  the  leakage  of  valves  and  the  initial  condensa 
tion  of  steam  in  the  cylinder,  the  actual  consumption  is  somewhat 
greater.  The  excess  varies  considerably  and  makes  all  results  so 
obtained  of  little  value.  In  our  calculation  we  used  one  diagram 
only,  that  for  the  head  end;  we  assumed  the  diagram  from  the 
crank  end  to  be  the  same.  The  indicated  horse-power  for  each  end 
should  be  computed,  as  it  is  subject  to  considerable  variation.  In 
the  above  formula  the  average  mean  effective  pressure  should  be 
used.  The  results  of  calculations  of  steam  consumption  from 
indicator  cards  are  so  unreliable  that  engineers  place  little  depend- 
ence upon  them.  The  results  may  be  used  as  a  basis  for  estimates, 
but  for  accurate  knowledge,  an  engine  test  must  be  resorted  to 

EXAMPLES  FOR  PRACTICE. 

1.  Given  the  following  data  to  find  the  M.  E.  P.     Area  of 
card  2.79  square  inches,  length  of  card  3.1  inches,  scale  of  spring 
50.  Ans.  45  pounds. 

2.  Steam  enters  a  cylinder  at  a  pressure  of  210  pounds  by 
gage  and  leaves  at  3  pounds  pressure   (absolute).     What  is  the 
thermal  efficiency?  Ans.  29  per  cent. 

3.  An  engine  has  a  B.  H.  P.  of  78.90.      The  I.  H.  P.  is 
86.73.     What  is  the  mechanical  efficiency.  Ans.  91  per  cent. 

4.  An   engine    develops    149.97    I.    II.    P.      If   the    piston 
diameter  is  17|  inches,  the  stroke  30  inches,  and  the  M.  K.  P.  40 
pounds,  what  is  the  speed  ?  Ans.  100  revolutions  per  minute. 

5.  The  theoretical  M.  E.  P.  of  a  triple  expansion  marine 
engine  is  48  pounds.     What  is  the  probable  actual  M.  E.  P.? 

0.     Tlu3  cylinders  of  a  triple  expansion   are  12,  )>(),  and  75 
inches  in  diameter  respectively.     The  stroke  is  24  inches.     The 


INDICATORS. 


57 


mean  effective  pressures  from  the  cards  were  118.4,  51.6  and 
19.84  pounds  respectively.  What  is  the  I.  IT.  P.  when  the  speed 
is  125  revolutions  per  minute?  Ans.  2,050  I.  H.  P. 

7.  Find  the  steam  consumption  from  the  following  card. 
The  engine  was  running  at  75  revolutions  per  minute,  and  devel- 
oping 230  I.  H.  P.  when  the  card  was  taken.  A  GO-pound  spring 
was  used,  and  the  M.  E.  P.  for  this  end  was  40  pounds.  Assume 
crank-end  card  to  be  the  same  as  the  head  end. 

Ans.  22  pounds  (about). 


H 


\ 


Suggestion:  First  draw  a  line  .3  inch  above  the  atmospheric  line, 
then  reduce  this  ordinate  to  absolute  pressure,  and  use  formula  at 
the  bottom  of  page  55. 


VlfcBiaLA 

Of  THE 

UNIVERS 

OF 


INDEX 


Part  T — VALVE   GEARS;    Part  II — STEAM   ENGINE  INDICATORS 


Part  Page 

Adjustable  eccentrics I,  54 

Admission I,  9 

Admission  line II,  36 

Area  of  cards,  to  find II,  31 

Atmospheric  line II,  36 

Back  pressure  line II,  37 

Balanced  valves I,  41 

Brake  horse-power II,  28 

Bridge,  width  of I,  31 

Brown  releasing  gear I,  68 

Clearance  line II,  38 

Combined  diagrams II,  44 

Compensation  of  cut-off I,  14 

Compound  engines,  horse-power  of II,  47 

Compression I,  9 

Compression  curve II,  38 

Corliss  valve  setting , I,  72 

Crosby  indicator :  .  .  . II,  12 

inserting  spring II,  14 

Cut-off.... .- I,  9 

compensation  of I,  14 

point  of I,  31 

Double-ported  valve I,  39 

Double  valve  gears I,  58 

Meyer I,  59 

Drop  cut-off  gears I,  64 

Brown  releasing. I,  68 

Greene I,  70 

Reynolds-Corliss I,  65 

Sulzer I,  71 

Eccentric I,  4 

Eccentrics,  adjustable I,  54 

Engine,  putting  on  center I,  36 

Engine  constants,  table II,  27 

Exhaust  lap I,  10 

Exhaust  line -.:••••  II,  37 

Exhaust  port,  width  of I,  31 


II  INDEX 


Part  Page 

Expansion  curve II,  37 

Gears 

Hackworth. '. I,  50 

Joy I,  52 

Marshall I,  52 

Walschaert. I,  53 

Goochlink... . I,  43 

Greene  gear I,  70 

Hackworth  gear I,  50 

Horse-power  of  compound  engines II,  47 

Indicator  cards II,  48 

Indicator  diagram,  theoretical II,  35 

Indicator  diagrams,  to  take II,  22 

Indicators. II,  9 

Crosby II,  12 

Tabor II,  15 

Thompson.. .'. II,  10 

Inequality  of  steam  distribution I,  12 

Joy  gear I,  52 

Lead,  angle  of I,  11 

Lead  of  engines .  I,  32 

Link  motion,  Stephenson I,  42 

Marshall  gear .' I,  52 

Mechanical  efficiency  of  engine II,  31 

Meyer  valve I,  59 

design  of I,  60 

Negative  lap I,  15 

Pantograph II,  19 

Pencil  mechanism  of  Tabor  indicator II,  15 

Piston  valve I,  33 

Plain  slide  valve. I,  3 

Planimeter II,  33 

to  use II,  34 

Point  of  cut  off II,  37 

Point  of  exhaust  closure II,  37 

Point  of  release II,  37 

Power  of  engine II,  3 

Prony  brake II,  28 

Radial  valve  gears. . .  . •• I,  49 

Reducing  motion '. II,  18 

Release I,  9 

Reynolds-Corliss  gear I,  65 

Rocker '..  .1,  15 

Rope  brake. II,  29 

Safety  cam I,  68 

Slide  valve I,  3 

designing I,  22,  30 

modifications  of . .  . I,  38 


INDEX  III 


Part  Page 

Steam  consumption    II,  52 

Steam  distribution,  inequality  of I,  12 

Steam  lap I,  10 

Steam  line.... II,  36 

Steam  pipe,  area  of I,  30 

Steam  port,  width  of I,  31 

Stephenson  link  motion I,  42 

Sulzer  gear I,  71 

Tables 

engine  constants.. II,  27 

laps,  travel,  and  angular  advance,  effect  of  changing I,  22 

Tabor  indicator II,  15 

atmospheric  line,  change  of  location  of II,  16 

attaching  to  engine II,  17 

care  of II,  17 

pencil  mechanism II,  15 

springs II,  16 

Theoretical  card,  to  draw II,  39 

Theoretical  indicator  diagram II,  35 

Thermal  efficiency II,  35 

Thompson  indicator .  II,  10 

care  of II,  12 

change  springs II,  11 

Three-way  cock II,  18 

Trick  valve I,  41 

Valve  diagrams I,  16 

Zeuner's I,  16 

Valve  gears. 7 ;  .  . .  I,  3 

double I,  58 

problems  on , I,  22 

Valve  setting I,  35 

Valves 

balanced I,  41 

double-ported I,  39 

with  lap I,  7 

without  lap f . .  .  . 'I,  5 

piston I,  38 

plain  slide I,  3 

setting  for  equal  cut-off. I,  37 

setting  with  equal  lead .  I,  37 

trick I,  41 

Walschaert  gear I,  53 

Watt's  diagram  of  work II,  8 

Work  of  engine,  definition  of II,  3 

Zero  line  of  pressure II,  38 

Zeuner's  diagram I,  16,  55 


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how  to  estimate  intelligently.  Price... $1.00 

CONTRACTS  AND  SPECIFICATIONS.  By  James  C. 
Plant.  130  PP  ,  fully  illustrated.  Forms  of 
public  and  private  contracts,  specifications, 
bonds,  etc.;  duties  and  responsibilities  of 
Architects,  Contractors,  and  Owners 
Price $1.00 

STAIR-BUILDING  AND  STEEL  SQUARE.  By  Hodgson 
and  Williams.  130  pp.,  180  illus.  Only  up-to- 
date  work  on  these  subjects.  Price $1.00 

VALVE  GEARS  AND  INDICATORS.  By  Leland  and 
Dow.  150  pp.,  105  illus.  Two  books  in  one. 
Types  of  valves,  gears,  etc.,  fully  explained. 
Price $1.00 


STRENGTH  OF  MATERIALS.  By  E.  R.  Maurer.  140 
pp.,  58  illus.  For  Architects*,  Builders.  Steel 
and  Concrete  Workers.  Enables  one  to 
avoid  mistakes.  Price $  1 .00 

THE  ELECTRIC  TELEGRAPH.  By  Thorn  and  Collins. 
150  pp.,  81  illus.  Carries  along  by  easy  steps 
to  complete  mastery  Multiplex  and  Wire- 
less telegraph  explained.  Price $1.00 

MECHANICAL  DRAWING.     By  E.  Kenison.     160pp., 
140  illus.      Complete  course  in    projections, 
shade  lines,  intersections  and  developments, 
lettering,    with     exercises    and    plates. 
Price $  1 .00 

POWER  STATIONS  AND  TRANSMISSION.  By 
G.  C.  Shaad.  160  pp.,  43  illus.  For  Electrical 
Workers.  Up-to-date  practice.  Price  $1.00 

PATTERN  MAKING.  By  James  Ritchey.  150  pp., 
250  illus.  For  Wood  and  Metal  Workers  and 
Molders.  Methods  of  building  up  and  fin- 
ishing, fully  described.  Price $1.00 

SURVEYING.  By  Alfred  E.  Phillips.  200  pp  ,  133 
illus.  For  Civil  Engineers  and  Students.  All 
details  of  field  work  explained.  Price  $1.50 

STEEL  CONSTRUCTION.  By  E.  A.  TUCKER.  300pp., 
275  illus.  Covers  every  phase  of  the  use  of 
steel  in  structural  work.  Based  on  actual  ex- 
perience, special  tests,  etc.  For  Architects, 
Bridge  Builders,  Contractors,  Civil  Engineers. 
Price $  1 .50 

BUILDING  SUPERINTENDENCE.  By  E.  Nichols 
200  pp.,  250  illus.  Costly  mistakes  occur 
through  lack  of  attention  at  proper  time, 
hurtful  to  Owner  and  discreditable  to  Archi- 
tect and  Builder.  Gives  thorough  knowledge 
of  methods  and  materials.  Price $1.50 

ARCHITECTURAL  DRAWING  AND  LETTERING.  By 
Bourne,  von  Hoist  and  Brown.  200  pp.,  55  draw- 
ings. Complete  course  in  making  working 
drawings  and  artistic  lettering  for  architec- 
tural purposes.  Price $  1.50 

MACHINE  SHOP  WORK.  By  F.  W.  Turner.  200  pp., 
200  illus.  Meets  every  requirement  of  the 
shopman,  from  the  simplest  tools  to  the  most 
complex  turning  and  milling  machines. 
Price $  1.50 

TOOL  MAKING.  By  E. R.  Markham.  200  pp., 325  illus. 
How  to  make,  how  to  use  tools.  Profusely 
illustrated.  Price $  1 .50 

MACHINE  DESIGN.  By  C.  L.  Griflin.  200  PP., 
82  designs.  Written  by  one  of  the  foremost 
authorities  of  the  day.  Every  illustration 
represents  a  new  device  in  machine  shop 
practice.  Price $1.50 


These  volumes  are  hand 
prepaid  to  any  part  of  thejyo-.^ . 
or  Registered  Letter. 


8 


red  art  Vellum  de  Luxe,  size  6%  x  9%  inches.    Sent 
price.    Remit  by  Draft,  Postal  Order,  Express  Order, 


AMERICAN  SCHOOL  OF  CORRESPONDENCE,  CHICAGO 


